Re: RE: [Repeater-Builder] Duplexers
Bob, It is better to have someone so one can set down and talk to. This e-mail is great for bringing the world together, but face to face is much better except for most of the ugly Hams I hang out with. The problem with a tuner is the feedline losses, but better than no tuner at all unless got resonant antennas. On HF this is harder to do if one moves about. Putting the tuner at the antenna is a solution, but then get into remote application. Some tuners are automatic and tune whenever they see higher than say 1.5:1 SWR. 73, ron, n9ee/r From: Bob Dengler [EMAIL PROTECTED] Date: 2007/08/30 Thu PM 12:53:11 CDT To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Duplexers OK, after talking to a senior RF engineer at lunch here at work I think I understand what's going on. The part that threw me was having the matching circuit in the middle of the feedline the fact that any reflected power from the load MUST be totally re-reflected back by the matching circuit, otherwise there would be power reflected back to the TX, which by definition does not occur in this example. Because of the multiple re-reflections between the matching circuit load resulting in multiple waves back forth within that coax section, typical single-wave thinking doesn't apply. I guess it's a useful way to illustrate why coax gets lossier if you use a tuner far from the antenna. Bob NO6B Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
Re: RE: [Repeater-Builder] Duplexers
Jeff, I have Reflections II. Walt does good job explaining transmission lines. The only problem I had was all the complaining he did about other authors and articles. Some was good, but seemed to get carried away times, hi. I think the book is $19.95. One of the Ham mag has it, think CQ or World Radio...def not QST. 73, ron, n9ee/r From: Jeff DePolo [EMAIL PROTECTED] Date: 2007/08/27 Mon PM 05:11:42 CDT To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Duplexers As a side note for this discussion, I think Jeff's doing a great job of explaining how transmission line theory works... I try... For those that want to dive in a lot further (e.g. Do the math), the ARRL Antenna Book has a whole section dedicated to this topic, and it's written well enough that a mathematical dolt like myself can still follow the concepts and not scratch my head at the math. Another good, readily-available book is Reflections by Walt Maxwell W2DU. My copy is old; he put out an updated edition Reflections II later. It's a good read, but has one drawback (to me anyway). A lot of what he discusses relates to pi matching networks common in tube HF rigs. You have to keep in mind that a lot of the myths he dispels don't always translate directly to the world of VHF/UHF solid state amplifiers, but theory behind what he preaches is dead nuts on. --- Jeff Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
RE: RE: [Repeater-Builder] Duplexers
Your typical swr meter does not measure voltage on the line. It is measuring a combination of voltage and current. By just measuring voltage it is impossible to tell which is forward and which is reflected. By using a slotted line you can find voltage min and max but you must have a line that is at least 1/4 wavelength long. This is impractical at HF frequencies and cumbersome at VHF. 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Ron Wright Sent: Wednesday, August 29, 2007 9:00 AM To: Repeater-Builder@yahoogroups.com Subject: Re: RE: [Repeater-Builder] Duplexers I think he quote needs little correction...âWhen you are using a VSWR meter you are measuring voltageS, not just one voltage, hi. You are measuring voltage ratios. The SWR reading due to losses changes when one moves closer or farther away from the end of a feedline. The power going out is attenuated, then the load reflects a portion of this back and gets attenuated again and the reflected is measured. Moving closer increases the power to the load and also increases the reflected read at the source showing a higher SWR. Lengthing the cable does the opposite. This is why one can have say 500 ft of RG58 at 450 MHz completly open at the load end and the swr might read 1.5:1 at the source. Also for long feedlines with antennas can give deceptive readings at the source. SWR at the load is much more real. 73, ron, n9ee/r From: R. K. Brumback [EMAIL PROTECTED] Date: 2007/08/27 Mon PM 12:25:59 CDT To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Duplexers Quote from Jesse: âWhen you are using a VSWR meter you are measuringvoltage, if you move the meter to a different spot on the cable, the voltage isdifferent, therefore it gives you a different reading.â This now makes more sense to me as I once saw a feed line demonstrationwith voltage and current sleds showing the difference at different points alongthe line. At some places the voltage was null (as with any sine wave). I donâtsee how this could happen at the antenna port of a transmitter unless it wasmicrowave as the cabling from the tuner to the output connector is not near ½ wave. Also to Alan, I appreciate your sympathy for us âlittle peopleâbut I do find this very interesting. And as you can see, the experts sometimesneed a tune up. Randy W4CPT -Original Message- From: [EMAIL PROTECTED]:Repeater- [EMAIL PROTECTED] OnBehalf Of Jesse Lloyd Sent: Monday, August 27, 2007 12:48 PM To:Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder]Duplexers The length of coaxdoesn't effect impedance. Trimming the coax effects what is read on theVSWR meter because what is actually happening is that there is an interferencepattern created when you have a mismatch on the end of feedline. This patternis sinusoidal and changes in voltage and current along the line, in 1/2 waveperiods. You will find max voltage peaks and min voltage peaks. Also current will go up and down too. When you are using a VSWR meter youare measuring voltage, if you move the meter to a different spot on the cable,the voltage is different, therefor it gives you a different reading. Now if you put a voltage null at your transmitter, what would happen? Normally with high SWR your transmitter will get hot because its dissipating thereflected power into its heatsink. If you put it at a voltage null, Iwould suspect that the SWR would not get dissipated by the transmitter as muchas if you put it at a voltage peak. The standing waves are still there,there is still a mismatch, you will get the same power out, but its just notgoing to hurt your transmitter as much because of the heat. The only time coax length makes a difference to power out is if your using itin a matching stub, or a matching section ie. if you take 1/4 wave of 75 ohmcable put it on the end of 50 ohm cable you will get a match with a 112.5 ohmload. You make an interesting point though, why does the cabling of duplexer's needto be a certain length. I would suspect that its because they are loopedand make an inductor. This then is part of the LC filtering, and changing thelength effects L. But I could be wrong on that. Jesse On 8/27/07, R. K. Brumback [EMAIL PROTECTED]wrote: I have heardthis point argued for years. Does trimming the coax affect theSWR? If the length of coax has an affect on impedance, then howcould it not affect power out? We strive to maintain 50 ohms at the tail of alldevices to match the end load. GE puts matching networks in their Mastr II's. Ihave taken a MFJ-259 and soldered a PL259 only at one end and then startedtrimming the coax down and watched the impedance change significantly with eachcut. Duplexers come with precise lengths of cabling. I have heard thattrimming coax only fools the meter. Not being
Re: RE: [Repeater-Builder] Duplexers
Gary, Transforming from a load to 50 Ohm resistive depends on the load impedance. Same for other impedances. Going from a load to another resistive load will always be a the ratio of N:50. So 50 Ohm coax could transform say a 75 OHM load to a 75:50 or 50:75 ratio. 75 Ohm load with 1/4 wave 50 Ohm coax to 33 Ohms, but not anything. At 1/2 wave back to 75 Ohm. In between would be R with j component, inductive or capacitive depending on length. A Smith Chart shows this. So converting the load to what you want would normally require a different cable impedance. The wiring harnesses used in multi-element antennas like the DB224 4 bay dipole antenna use cable type to convert each antenna load to a 50 Ohm input. In this case there are actually 3 harnesses, one for each of 2 dipole sets and then a 3rd to take these two to one input. I think 92 or 62 Ohm cable is used, but not sure. 73, ron, n9ee/r From: Gary Schafer [EMAIL PROTECTED] Date: 2007/08/27 Mon PM 10:52:11 CDT To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Duplexers   From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jesse Lloyd Sent: Monday, August 27, 2007 9:39PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder]Duplexers  If your coax is the sameimpedance as your transmitter, but different than your load, can it still be atransformer though? Is it possible to transform a load that isn't 50 ohmsto 50 ohms using 50 ohm coax?  Yes italways acts as a transformer when the load impedance is not the same as thecoax impedance. You cannot transform any impedance to 50 ohms with a 50 ohm cable. You can transformto something above or below the 50 ohm cable impedance. Thereason changing the length of the coax to a transmitter helps sometimes, eventhough the transformation of impedance is not to 50 ohms, is that thetransmitter may see an impedance that it is happier with than what the originaltransformed impedance was. Yes your right VSWR is the ratio between Vmax and Vmin, node and anodes, of theinterference pattern caused by standing waves. Even still there is apoint where the voltage is at a minimum on the line. What happens if thatpoint is at the transmitters output... does it help keep the heat down in thetransmitter due to high SWR?  It doesnâtmatter where the min and max are on the line. The same amount of reflectedpower will be seen at any point. Reflected power does NOT get back into thetransmitter. It gets re-reflected back towards the antenna when it reaches thetransmitter circuits. If youhave a 100 watt transmitter with 10 watts reflected from the load yourwattmeter will read 110 watts forward and 10 watts reflected. The extra 10watts forward power comes from the 10 watts that is reflected from the load andre-reflected at the transmitter. The re-reflected power adds to the original100 watts forward power for a total of 110 watts forward power. All of the 100watts eventually gets radiated by the antenna. This is of course disregardingany line loss which would lower the reflected power indication by the amount ofline loss. Line loss would also claim a portion of the re-reflected power too. If youhave two watt meters and an antenna matching device you can put one wattmeter betweenthe transmitter and the matching device and tune it for minimum reflected poweron the first meter. Then with a second meter between the tuner and themismatched load you can see the second wattmeter that is reading the reflectedpower. The second wattmeter will have a higher forward power reading than thefirst due to the added re-reflected power.  With amismatched load the transmitter may run hotter because it is under oroverloaded due to the non 50 ohm load that it is seeing but it is notdissipating any of the reflected power. Many solid state transmitters aresensitive to reactive loads and may draw more current because of this.  73 Gary K4FMX On 8/27/07, GarySchafer [EMAIL PROTECTED]wrote: VSWR (voltage standingwave ratio) will be the same at any point on a transmission  line. Theimaginary standing wave does not move as the forward and reflected power does.The voltage standing wave ratio is the ratio of the forward voltage to thereflected voltage at a given point on the line. As you move up or down the linethe forward voltage will change and so will the reflected voltage but the ratioor difference between the two will work out to the same value. Thus the termstanding wave. The wave appears to stand still on the line as itoscillates up and down in a sin wave manor.  As Jeff has said theimpedance shown to the transmitter will be different with different lengths oftransmission line only if the load is not a perfect 50 ohms assuming a 50 ohmline. With a load that does not match the line the line operates as animpedance transformer. Think about what a quarter wave length line looks likewith a
