Re: [Elecraft] K3 - Numeric SWR Display
On Wed, 17 Sep 2008, Bill W5WVO wrote: Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Alternately, feeding through one or more half-waves of coax will give you good accuracy while letting you get far enough away from the antenna to not affect the measurement. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
Alternately, feeding through one or more half-waves of coax will give you good accuracy while letting you get far enough away from the antenna to not affect the measurement. Only if the feedline has zero loss. Force 12 are known for making their antennas look better by specifying the SWR through 100 feet of feedline. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jessie Oberreuter Sent: Thursday, September 18, 2008 2:42 AM To: Bill W5WVO Cc: LIST - elecraft; Cranz Nichols Subject: Re: [Elecraft] K3 - Numeric SWR Display On Wed, 17 Sep 2008, Bill W5WVO wrote: Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Alternately, feeding through one or more half-waves of coax will give you good accuracy while letting you get far enough away from the antenna to not affect the measurement. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
Since I don't plan on hanging off the side of the tower to operate my station, isn't it more prudent to tune the entire system, flaws, feedline and all if my main concern is what SWR my equipment, in my shack, sees? Charlie KI5XP -Original Message- From: [EMAIL PROTECTED] [mailto:elecraft- [EMAIL PROTECTED] On Behalf Of Jessie Oberreuter Sent: Thursday, September 18, 2008 1:42 AM To: Bill W5WVO Cc: LIST - elecraft; Cranz Nichols Subject: Re: [Elecraft] K3 - Numeric SWR Display On Wed, 17 Sep 2008, Bill W5WVO wrote: Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Alternately, feeding through one or more half-waves of coax will give you good accuracy while letting you get far enough away from the antenna to not affect the measurement. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
-Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Charles Morrison Since I don't plan on hanging off the side of the tower to operate my station, isn't it more prudent to tune the entire system, flaws, feedline and all if my main concern is what SWR my equipment, in my shack, sees? No it isn't. If you're using 50 ohm coax and an antenna that is not 50 ohms resistive, you'll set up standing waves in the coax which will result in signal loss. Having a tuner at the rig will protect the rig from the effects of the standing waves, but the line loss will still be there. Far better is to have the tuner at the actual antenna feed point (or to adjust the antenna to be 50 ohm resistive). Then you'll have a happy rig AND minimal loss in the coax. - Keith N1AS - - K3 711 - ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
That was the point I was trying to make originally. However, to be fair, the feedline loss generated by SWR is significant only where the loss is substantial enough to make a meaningful difference. At HF, particularly the lower bands, the SWR loss is, from a practical standpoint, insignificant. However, I'm a VHF weak-signal operator, and I tend to think in those terms. Having a non-reactive load at the feedpoint is important at VHF/UHF, as feedline losses due to SWR can be significant on both transmit and receive. Bill W5WVO - Original Message - From: Darwin, Keith [EMAIL PROTECTED] To: LIST - elecraft elecraft@mailman.qth.net Sent: Thursday, September 18, 2008 8:40 AM Subject: RE: [Elecraft] K3 - Numeric SWR Display -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Charles Morrison Since I don't plan on hanging off the side of the tower to operate my station, isn't it more prudent to tune the entire system, flaws, feedline and all if my main concern is what SWR my equipment, in my shack, sees? No it isn't. If you're using 50 ohm coax and an antenna that is not 50 ohms resistive, you'll set up standing waves in the coax which will result in signal loss. Having a tuner at the rig will protect the rig from the effects of the standing waves, but the line loss will still be there. Far better is to have the tuner at the actual antenna feed point (or to adjust the antenna to be 50 ohm resistive). Then you'll have a happy rig AND minimal loss in the coax. - Keith N1AS - - K3 711 - ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
That's true. Indeed, if you have an antenna tuner (built-in automatic or external manual) you don't even need to do that. That's what made them so popular in recent years; they fix impedance problems at the rig end without fiddling around with the antenna at all. The issue then becomes feed line loss. It can get quite high, especially at the higher frequencies and with longer lengths of coax. It's not unusual to throw away 50%, 75% or more of your RF as heat along a coaxial line that way. But, if your antenna is designed to provide a decent match, or if you've done some adjustments 'on the ground' that suggest you're at least approximately correct, you're not likely to experience such drastic losses even though things will change when the antenna is raised to its final position. Doing measurements up on a tower is one of the major reasons for the popularity of the modern antenna analyzers. They're self contained and small enough you can carry one up to the feed point. Most Hams only need to do that at rare intervals, which is why many Ham clubs have a club analyzer everyone's contributed to buying so members can borrow it on occasion. A much cheaper approach is, as Bill says, to cut some coax to a multiple of 1/2 wave, electrically. Then the impedance you see at the end on the ground will be the impedance at the antenna. The issue there is how many times do you want to climb the tower? Perhaps a buddy at the rig on the ground and you in the air with some HT's is the answer to that. Ron AC7AC -Original Message- Since I don't plan on hanging off the side of the tower to operate my station, isn't it more prudent to tune the entire system, flaws, feedline and all if my main concern is what SWR my equipment, in my shack, sees? Charlie KI5XP ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
Hi Charlie: Since I don't plan on hanging off the side of the tower to operate my station, isn't it more prudent to tune the entire system, flaws, feedline and all if my main concern is what SWR my equipment, in my shack, sees? 'Tuning' things so the rig sees what it wants is not always the best... since things could be REALLY BAD at the antenna end, but you'd never know it at the radio end. You wan the BEST MATCH available AT THE ANTENNA END so you'll get maximum (most efficient) RF power transfer from the feedline to the antenna. Once you get this, you can worry about performing and additional matching at the TX end. Using a transmatch, you can make darn near any level of mismatch 'appear' to be a 1:1 SWR at the transmitter, but if the match AT THE ANTENNA is not acceptable, you won't be getting the most efficient transfer of RF to the antenna... e.g. you'll be heating your feedline with the RF that was to go into the antenna... a place where it'll do little good. FIRST - Ensure that the feedline-to-antenna match is appropriate for good power transfer. THEN - Ensure that the match between your feedline and the transmitter is appropriate for good power transfer from the transmitter to the feedline and ultimately to the antenna. 73, Tom Hammond N0SS ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
On Thu, 18 Sep 2008 09:35:45 -0600, Bill W5WVO wrote: as feedline losses due to SWR can be significant on both transmit and receive It depends on your definition of significant and the degree of mismatch. This is another one of those old wives' tales that is a wild exageration of reality. A graph of the loss due to mismatch has appeared in every version of the ARRL Handbook and the ARRL Antenna Book for many years. It shows that the WORST CASE additional loss for a 1.5:1 VSWR is 0.18 dB, no matter how much the matched loss of the line! That worst case number is 0.5 dB for a 2:1 VSWR and 1.2 dB for 3:1 VSWR. If the loss in the line for a perfect match were 1 dB, the additional loss due to SWR is about 0.05dB for 1.5 VSWR, 0.2 dB for 2:1, and 0.46dB for 3:1. You've got to have a VSWR of 5:1 for there to be 1 dB of additional loss due to mismatch! Yes, the loss in any transmission line causes the VSWR to come closer to 1:1 as we move along the line from the antenna to the transmitter. The lossier the line, the greater this effect. BUT -- coax of reasonable size and quality is NOT very lossy on the HF bands. Decent RG8X (Belden 9258 or LMR240) has only about 1 dB/100 ft on 20M, and less than 0.5dB/100ft on 80M. Loss rises t about 1.6dB on 10M, and 2 dB/100 ft on 6M. The best RG8s have roughly one-third that loss. This graph is in Chapter 19 of my 2002 ARRL Handbook, and Chapter 24 of the ARRL Antenna Book. Both publications also show the equation from which the graphs are derived, and the ARRL Antenna Book comes with a calculator (by N6BV) that will compute the loss in the line for any length of line, frequency, and value of termination impedance that you plug in, based on its database of loss for many popular coax cables (Belden, LMR, and a few others). You can also plug in the VSWR or complex impedance you've measured at the transmitter and the length of the line, and it will compute the line loss and the VSWR at the antenna. The bottom line is that unless your antenna is badly mismatched (far off resonance or broken), the additional loss due to VSWR on the HF bands is truly insignificant. Now, I DO use big coax (RG8 and RG11) on my high wire dipoles that are up about 110 ft (and the lines are about 140 ft). I do that because I am contesting (high duty cycle) with legal power and over rather wide bandwidths (a single dipole for all of 80/75 and for 160M), AND because I use the 80/40 fan dipoles on 30M, 17M, and 12M. The fact that I'm using the big coax saves me a dB or so in these far off resonance conditions, and allows them to work pretty well! 73, Jim Brown K9YC ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
Excellent point Joe. The current QST magazine that features a short article about running open wire or window line to coax outside the house and then using a short length of coax to reach the rig. The author noted that with a short run of 10 feet of the coax he uses, the losses with a 10:1 SWR would be about 1 dB at 28 MHz. That sounds good assuming the coax losses and SWR are really that low in worst case. But what he missed was that he used a choke balun between the coax and feed line that consists of an additional 22 feet of coax wound up in a coil to keep RF off of the outside of the coax shield and so out of the shack. So he really has 32 feet of coax between the rig and the open wire feed line. If he original assumptions about maximum SWR and losses were correct, he's really losing about half of his transmitter power (3 dB) in that coax link and balun. Ron -Original Message- Alternately, feeding through one or more half-waves of coax will give you good accuracy while letting you get far enough away from the antenna to not affect the measurement. Only if the feedline has zero loss. Force 12 are known for making their antennas look better by specifying the SWR through 100 feet of feedline. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
On Thu, 18 Sep 2008 09:44:42 -0700, Ron D'Eau Claire [EMAIL PROTECTED] wrote: That's true. Indeed, if you have an antenna tuner (built-in automatic or external manual) you don't even need to do that. That's what made them so popular in recent years; they fix impedance problems at the rig end without fiddling around with the antenna at all. The issue then becomes feed line loss. It can get quite high, especially at the higher frequencies and with longer lengths of coax. It's not unusual to throw away 50%, 75% or more of your RF as heat along a coaxial line that way. But, if your antenna is designed to provide a decent match, or if you've done some adjustments 'on the ground' that suggest you're at least approximately correct, you're not likely to experience such drastic losses even though things will change when the antenna is raised to its final position. Doing measurements up on a tower is one of the major reasons for the popularity of the modern antenna analyzers. They're self contained and small enough you can carry one up to the feed point. Most Hams only need to do that at rare intervals, which is why many Ham clubs have a club analyzer everyone's contributed to buying so members can borrow it on occasion. A much cheaper approach is, as Bill says, to cut some coax to a multiple of 1/2 wave, electrically. Then the impedance you see at the end on the ground will be the impedance at the antenna. The issue there is how many times do you want to climb the tower? Perhaps a buddy at the rig on the ground and you in the air with some HT's is the answer to that. Ron AC7AC -Original Message- [snip] I bought an AIM 4170 analyzer at the Ft Worth Hamfest this year and love it. When measuring antennas with it I calibrate the coax before making the antenna measurement with a piece of 100 ft RG213 on a ten foot mast. After I have done that I can measure the antenna as though the analyzer is attached to the terminals at the antenna. Then I put the antenna on the tower and measure it again with the same calibrated coax to find out what changed at the additional height. If it checks ok I connect the LMR600 to the antenna and call it good. Tom, N5GE - SWOT 3537 - Grid EM12jq Those who would give up Essential Liberty to purchase a little Temporary Safety deserve neither Liberty nor Safety An excerpt from a letter written in 1755 from the Assembly to the Governor of Pennsylvania. Support the entire Constitution, not just the parts you like. http://www.n5ge.com http://www.eQSL.cc/Member.cfm?N5GE ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
Gary, The SWR is being displayed when you press TUNE. The tuning power can be set at TUN PWR, set it higher than 2 Watts. Steef PA2A K3 1184 - Original Message - From: Cranz Nichols [EMAIL PROTECTED] To: LIST - elecraft elecraft@mailman.qth.net Sent: Wednesday, September 17, 2008 11:53 PM Subject: [Elecraft] K3 - Numeric SWR Display I need to adjust the gamma matches on my 10/15M delta loop. Is there a way to bypass the internal ATU and have the K3 display the SWR it sees numerically? Should dig out my old SWR bridge? cln WB5BKL K3 #231 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Bill W5WVO - Original Message - From: Cranz Nichols [EMAIL PROTECTED] To: LIST - elecraft elecraft@mailman.qth.net Sent: Wednesday, September 17, 2008 3:53 PM Subject: [Elecraft] K3 - Numeric SWR Display I need to adjust the gamma matches on my 10/15M delta loop. Is there a way to bypass the internal ATU and have the K3 display the SWR it sees numerically? Should dig out my old SWR bridge? cln WB5BKL K3 #231 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
I should add that you CAN adjust an antenna for minimum SWR at the transmitter, but when you do that, you are in all likehood including some non-zero feedpoint reactance in the net impedance, and this is being observed through the length of your transmission line, which then becomes part of the overall load the transmitter sees. If you subsequently change the length of the transmission line, you will no longer have Z=50 ohms at the transmitter. If you tune the antenna for TRUE resonance (zero reactance, R=50), then you can put any length of transmission line on it that you want to, and it will behave just the same. I know we all got along without the MFJ-259B for years, just going by guess and by gosh (or sweating over Smith charts), but now that we can actually tell what is happening in an antenna so easily, it's crazy not to use one. Beg, borrow, steal, or if necessary buy one, and learn how to use it. You won't regret it! Greatest thing since CW killed King Spark. :-) Bill W5WVO - Original Message - From: Bill W5WVO [EMAIL PROTECTED] To: Cranz Nichols [EMAIL PROTECTED]; LIST - elecraft elecraft@mailman.qth.net Sent: Wednesday, September 17, 2008 4:39 PM Subject: Re: [Elecraft] K3 - Numeric SWR Display Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Bill W5WVO - Original Message - From: Cranz Nichols [EMAIL PROTECTED] To: LIST - elecraft elecraft@mailman.qth.net Sent: Wednesday, September 17, 2008 3:53 PM Subject: [Elecraft] K3 - Numeric SWR Display I need to adjust the gamma matches on my 10/15M delta loop. Is there a way to bypass the internal ATU and have the K3 display the SWR it sees numerically? Should dig out my old SWR bridge? cln WB5BKL K3 #231 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
An adequate and not too expensive vector analyzer is the VA1 by Autek Research. It reads resistance and reactance (including its sign) directly and also has a function that automatically calculates the Z at your antenna even though you are measuring at the rig end of your transmission line. It is tiny and and battery operated and hence can be used on balanced lines without unbalancing things very much. I have no connection with the company, just a satisfied customer. Use google to find them. --Oliver Johns W6ODJ On 17 Sep 2008, at 3:52 PM, Bill W5WVO wrote: I should add that you CAN adjust an antenna for minimum SWR at the transmitter, but when you do that, you are in all likehood including some non-zero feedpoint reactance in the net impedance, and this is being observed through the length of your transmission line, which then becomes part of the overall load the transmitter sees. If you subsequently change the length of the transmission line, you will no longer have Z=50 ohms at the transmitter. If you tune the antenna for TRUE resonance (zero reactance, R=50), then you can put any length of transmission line on it that you want to, and it will behave just the same. I know we all got along without the MFJ-259B for years, just going by guess and by gosh (or sweating over Smith charts), but now that we can actually tell what is happening in an antenna so easily, it's crazy not to use one. Beg, borrow, steal, or if necessary buy one, and learn how to use it. You won't regret it! Greatest thing since CW killed King Spark. :-) Bill W5WVO - Original Message - From: Bill W5WVO [EMAIL PROTECTED] To: Cranz Nichols [EMAIL PROTECTED]; LIST - elecraft elecraft@mailman.qth.net Sent: Wednesday, September 17, 2008 4:39 PM Subject: Re: [Elecraft] K3 - Numeric SWR Display Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Bill W5WVO - Original Message - From: Cranz Nichols [EMAIL PROTECTED] To: LIST - elecraft elecraft@mailman.qth.net Sent: Wednesday, September 17, 2008 3:53 PM Subject: [Elecraft] K3 - Numeric SWR Display I need to adjust the gamma matches on my 10/15M delta loop. Is there a way to bypass the internal ATU and have the K3 display the SWR it sees numerically? Should dig out my old SWR bridge? cln WB5BKL K3 #231 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 - Numeric SWR Display
If you have 1:1 SWR at the transmitter, you must have a 50+j0 load at the other end of a 50 ohm coax, regardless of the line length. Using a Smith Chart will just show a dot at the center. (Of course, if you have a very lossy line, it could be a little off 50 ohms at the load.) This also assumes that you have a reasonable balun at the antenna, so the outside shield of the coax isn't a coupled part of the antenna that radiates. As for the MFJ-259B, it can be a great tool, but it can also give some pretty crazy readings, especially in the presence of other nearby transmitters, even when well out of band. I see postings almost every week on the Topband list about strange feed impedances of an inverted-L that are probably getting a 100 milliwatt signal from the local AM station. 73, Terry N6RY At 03:52 PM 2008-09-17, Bill W5WVO wrote: I should add that you CAN adjust an antenna for minimum SWR at the transmitter, but when you do that, you are in all likehood including some non-zero feedpoint reactance in the net impedance, and this is being observed through the length of your transmission line, which then becomes part of the overall load the transmitter sees. If you subsequently change the length of the transmission line, you will no longer have Z=50 ohms at the transmitter. If you tune the antenna for TRUE resonance (zero reactance, R=50), then you can put any length of transmission line on it that you want to, and it will behave just the same. I know we all got along without the MFJ-259B for years, just going by guess and by gosh (or sweating over Smith charts), but now that we can actually tell what is happening in an antenna so easily, it's crazy not to use one. Beg, borrow, steal, or if necessary buy one, and learn how to use it. You won't regret it! Greatest thing since CW killed King Spark. :-) Bill W5WVO - Original Message - From: Bill W5WVO [EMAIL PROTECTED] Sent: Wednesday, September 17, 2008 4:39 PM Subject: Re: [Elecraft] K3 - Numeric SWR Display Actually, looking at transmission line SWR at the transmitter is a poor way to adjust an antenna. To get it right, you need to use a complex impedance analyzer like the now-ubiquitous MFJ-259B (or equivalant instrument), and put it as close to the antenna feedpoint as possible. Adjust as close as you can get for Xc=0, Xl=0, R=50. You can't do that with an SWR bridge! :-) Bill W5WVO ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 - Numeric SWR Display
It is best to measure the SWR at the antenna. That ensures the actual SWR on the feed line (and so feed line losses) are as low as possible. Using an SWR bridge designed for 50-ohm lines will do the job. When the SWR bridge reads something close to 1:1 the reactance will, by definition, be close zero. If the resistive part of the impedance isn't 50 ohms, or if reactance is present, a match near 1:1 won't be possible. An analyzer is helpful when happens since it'll tell you what is off, but you can often determine that by simply checking the best match at another frequency (within the Ham band, of course). If the lowest SWR is lower at a higher frequency, there is inductive reactance. If the opposite is true, there is capacitive reactance. You may know that with a gamma match you adjust the length of the matching arm to hit the right resistive value then, because the gamma arm has length, and so inductance, you set the capacitor to compensate for that inductance to bring the reactance to zero. So really there's only two adjustments to get the reactance to zero and the resistance to 50 ohms. If you make the arm longer (move the shorting bar or whatever you're doing to adjust it) you'll raise the resistive value. Shorter will lower it. Each time you change the arm length you adjust the capacitor for minimum SWR. So you just work back and forth to reach an acceptably low SWR. Exact matches aren't important, especially since you have the KAT3. All you need to do is avoid too-high an SWR on the feeder. What is too high depends upon the length of the feed line and frequency, but in most amateur installations with 100 feet of decent coax there's little to tell between the losses caused by an SWR of 1:1 and 3:1 on a feed line. For example, common RG-8 coax with an SWR of 3:1 at 30 MHz only shows about 1 dB of loss. The loss is less as you reduce frequency and/or coax length. But, to understand what the SWR on the feed line really is, you need to measure it at the antenna (load), not at the source (rig). An SWR bridge will indicate lower at the rig end than at the antenna end due to the losses in the feed line. All the fancy gear, like analyzers, makes life easier and lets us do things more quickly, but they aren't required to install an effective, efficient antenna and they cost substantial dollars. Shoot, when I first got on the air, I and hundreds of thousands of Hams like me, put out good signals using efficient antennas without so much as an SWR bridge. Such exotic stuff as an SWR meter was still years in the future for most Hams! Our rigs offered tuning controls for the output network that allowed us to adjust them for proper mismatching, often over quite a range of load impedances. We didn't have to worry about mismatches until the feed line losses were so high as to throw away significant RF power. SWR only became really important with the advent of modern no tune transmitters. These transmitters use fixed-tuned output filters that match the impedance of the finals to 50 ohms. No tuning is required, but they require the load impedance to be close to 50 ohms, non reactive, or they reflect the mis-match to the finals which then run inefficiently and hot. The output filters as well no longer do their job to reduce spurious emissions correctly when faced with a severe impedance mis-match. With the first no-tune rigs, Hams become understandably paranoid about low SWR. Finals, both tube and transistors, tended let out their smoke very fast with high SWRs. That happened a lot as Hams brought their first no-tune rig home and hung it onto the feed line they had been using with the old, manually tuned output rigs, never realizing the 50 ohm coax was providing a load far removed from 50 ohms. The easiest, fastest solution for many Hams was to add a matching network, an antenna tuner, between the rig and the feed line. That's when companies like MFJ became popular with their lineup of tuners. We were back to twiddling knobs to adjust the output network, only now it was external to the rig. And that has led to the now-common built-in antenna tuner such as the KAT3 that does the same thing but automatically. So, once again, we no longer need be concerned with SWR unless it's so high it results in unacceptable feeder losses. We're right back were we started when coax first became popular in the late 1940's! The difference today is that we just push a button and hear the relays click instead of having to know which knob to turn while watching a meter to establish a proper match between the finals and the feed line. Ron AC7AC ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
