RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Think about it this way. If you made a dipole, would you cut one side 5% longer than the other? Maybe, if you had a reason for offset feed... but most offset Dipoles are not 5% different. I thought the subject was about 1/4 wave ground planes and not dipoles... they are not the same animal. That's my point. If there was a specific reason for making one side of the dipole exactly 5% longer (based on some pre-determination such as a computer model), then fine. But to generically say that all dipoles need to be cut with one side 5% longer than the other carries no more engineering support than saying that's how you should cut the radials on all groundplane antennas. I wasn't saying that there's never a reason to un-balance a balanced antenna. I was saying that a generic rule like make one side 5% longer in all cases lacks merit. Kinda along the same lines as always make the cable from the connector on the transmitter to the connector on the duplexer an even half-wave. As far as an elevated groundplane with a small number of radials, it does, in fact, behave more like a dipole than a ground-mounted (earthed) antenna, which is why I used the dipole as a simile in my rhetorical question. Why not make the vertical radiator 5% longer, and leave the radials exactly a quarter-wave, thereby saving on materials? Because a quarter wave ground plane doesn't work well with a beta match that way. Don't necessarily need a beta match for a groundplane. It can be regular series-fed, gamma, shunt, whatever. I'll email you direct... cheers, skipp I'll be on pins and needles! :-) Later. --- Jeff WN3A
RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
On Mon, 22 Feb 2010, Jeff DePolo wrote: Kinda along the same lines as always make the cable from the connector on the transmitter to the connector on the duplexer an even half-wave. The reason for doing that is that if the duplexer presents a short-circuit, said short-circuit won't appear at antenna port. Given the range of most duplexers, it would be unlikely that the cavity would cause such a situation to exist unless it were struck by lightning. -- Kris Kirby, KE4AHR Disinformation Analyst
RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
On Mon, 22 Feb 2010, Jeff DePolo wrote: Kinda along the same lines as always make the cable from the connector on the transmitter to the connector on the duplexer an even half-wave. The reason for doing that is that if the duplexer presents a short-circuit, said short-circuit won't appear at antenna port. Uwhat?
RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
On Mon, 22 Feb 2010, Jeff DePolo wrote: On Mon, 22 Feb 2010, Jeff DePolo wrote: Kinda along the same lines as always make the cable from the connector on the transmitter to the connector on the duplexer an even half-wave. The reason for doing that is that if the duplexer presents a short-circuit, said short-circuit won't appear at antenna port. Uwhat? I was thinking quarter-waves. If you have a tee, connect the antenna at the center and a duplexer to either side using quarter-wave cables, the effect I noted should occur, minimizing losses. -- Kris Kirby, KE4AHR Disinformation Analyst
RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
-Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- buil...@yahoogroups.com] On Behalf Of Kris Kirby Sent: Monday, February 22, 2010 5:03 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? On Mon, 22 Feb 2010, Jeff DePolo wrote: On Mon, 22 Feb 2010, Jeff DePolo wrote: Kinda along the same lines as always make the cable from the connector on the transmitter to the connector on the duplexer an even half-wave. The reason for doing that is that if the duplexer presents a short-circuit, said short-circuit won't appear at antenna port. Uwhat? I was thinking quarter-waves. If you have a tee, connect the antenna at the center and a duplexer to either side using quarter-wave cables, the effect I noted should occur, minimizing losses. -- Kris Kirby, KE4AHR Disinformation Analyst Well, at the output side of the duplexer that is what is happening already. The cables are a quarter wave length. The one from the transmit cans (to the antenna port) is a quarter wave length at the receive frequency and the one from the receive can (to the antenna port) is a quarter wave length at the transmit frequency. Since the receive can is tuned to the receive frequency, its output loop presents a short circuit to the transmit frequency. And since the cable going from that loop to the antenna T is a quarter wave length at the transmit frequency that short at the loop is seen as a very high impedance to the transmit frequency at the antenna T. The same thing happens on the transmit side of the affair but on the other frequency. That's how you get separation between the transmitter and receiver at the T junction. However, what Jeff was talking about was the cable between the TRANSMITTER and the duplexer input. His comment was tongue in cheek to make his point about the antenna. That cable in most cases can be any random length. There are times when a selected length will help the transmitter with the load that it sees due to out of band impedances that get presented to it. But you can not say that a certain length will be called for. 73 Gary K4FMX
Re: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Yes, I don't know why the db-201 has such ridiculously long radials, but that is how they were designed. Go figure. I would like to know why though if someone knows... 73 - Original Message - From: Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com To: Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com Sent: Sat Feb 20 21:31:38 2010 Subject: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? NORM KNAPP nkn...@... wrote: Make the vertical element 50.5 from tip of loop to flat plate where the radials rest.. The radials need to be 73 long... Works for me. 73 In the standard ground plane world... the radials should only need to be about 5% longer than the vertical element. However, longer than 5% is not going to be a huge problem as being less than 5% would be. s. - Original Message - From: Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com To: Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Sent: Sat Feb 20 16:20:07 2010 Subject: [Repeater-Builder] DB-201 Measurements for 6 Meters? Hi Folks, I looked at the cutting 'chart' here on r-b, but it didn't make much sense. The antennas that I have are at 47.2MHz, and resonate exactly at that frequency. I removed the loop, and the measurements are from Tip of curve to the end of each piece is 54 and 58. The cutting chart says 58.875 83.250. I'm missing something! Ideas? Thanks, Tim
Re: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
First, the DB-201 was not designed for tower side-mounting, so don't expect it to work well. That said, how far away from the tower will the antenna be? There's a real good chance that the VSWR will go sour when it's side-mounted. However, I have no personal experience with that antenna, but in all the low-band installations around here, I've never seen that antenna side-mounted. Chuck WB2EDV - Original Message - From: Tim Ahrens tahr...@swtexas.net To: Repeater-Builder@yahoogroups.com Sent: Saturday, February 20, 2010 11:59 PM Subject: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? Thanks Norm Skipp - I'll do some cutting tomorrow. Any clue what the pattern might be hung off the side of a wide spaced tower? (triangular about 15' between legs at the height the antenna will be). Thanks again! Tim W5FN
Re: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
By Design, Ground plane antennas are best top mounted, but of course its not always possible. That said, I have done a few and have found they tend to be fairly happy if mounting is at least 1/4 wave length away from the tower. See if you can picture this in the minds eye, 2 lengths of conduit, each threaded on one end and each with a T connector on the end, the 2 Ts tied together by a short length of conduit between them (Do this part first, about 12 should be enough) another 12 screwed in the top too attach the antenna. Screw the 2 longer pieces into this when you get on the tower. Mount the antenna and then slide it outwards. Check vswr and if needed play with what makes it the most happy by sliding it in or out. Then tighten down the clamps. I used a total of 4, 2 attached to 4 3/4 conduit clamps attached to the tower legs. It was relatively easy to slide it out to get it happy before tightening it up. I did this over 15 years ago and last time I drove past that area, I could see it was still up there. Even though that company is no longer there. But that tower with the antennas is still there. Makes you wonder if any of that is in use anymore. Good Luck! -Richard From: Chuck Kelsey wb2...@roadrunner.com To: Repeater-Builder@yahoogroups.com Sent: Sun, February 21, 2010 8:36:35 AM Subject: Re: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? First, the DB-201 was not designed for tower side-mounting, so don't expect it to work well. That said, how far away from the tower will the antenna be? There's a real good chance that the VSWR will go sour when it's side-mounted. However, I have no personal experience with that antenna, but in all the low-band installations around here, I've never seen that antenna side-mounted. Chuck WB2EDV - Original Message - From: Tim Ahrens tahr...@swtexas. net To: Repeater-Builder@ yahoogroups. com Sent: Saturday, February 20, 2010 11:59 PM Subject: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? Thanks Norm Skipp - I'll do some cutting tomorrow. Any clue what the pattern might be hung off the side of a wide spaced tower? (triangular about 15' between legs at the height the antenna will be). Thanks again! Tim W5FN
[Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Thanks for the info guys, I had planned on keeping it about 5 feet away from the tower for the TX antenna.. the RX antenna will be at the top of the mast, so it won't be an issue. (split site) Guess I'll do a bit of experimenting with the mounting. Tim
[Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
NORM KNAPP nkn...@... wrote: Yes, I don't know why the db-201 has such ridiculously long radials, but that is how they were designed. Go figure. I would like to know why though if someone knows... 73 As mentioned before, the radials on a conventional Ground Plane are about 5% longer as a general rule. The DB-201 is specified as available down to at least 35MHz and is often cut to fit at some customer locations. Decibel is probably not going to have that many different part numbers for the same radiator when trimming to desired length via a cutting charge is probably more practical for everyone. Just as a sidebar... I had a horrible time installing UHF DB-201 antennas in situations where any type of metal is nearby (IE inside San Francisco Skyscrapers). In fact I had to stop using them because in the close proximity of other metal object the UHF models are horrible PIM (Intermod) generators. I didn't seem to have any problems with the low and high band versions but I would recommend people not use the UHF Version unless it's mounted well free and clear of and nearby metal objects. It's just something about the UHF DB-201 design, size and application that's a very confirmed grunge generator. cheers, skipp - Original Message - From: Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com To: Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com Sent: Sat Feb 20 21:31:38 2010 Subject: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? NORM KNAPP nknapp@ wrote: Make the vertical element 50.5 from tip of loop to flat plate where the radials rest.. The radials need to be 73 long... Works for me. 73 In the standard ground plane world... the radials should only need to be about 5% longer than the vertical element. However, longer than 5% is not going to be a huge problem as being less than 5% would be. s. - Original Message - From: Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com To: Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Sent: Sat Feb 20 16:20:07 2010 Subject: [Repeater-Builder] DB-201 Measurements for 6 Meters? Hi Folks, I looked at the cutting 'chart' here on r-b, but it didn't make much sense. The antennas that I have are at 47.2MHz, and resonate exactly at that frequency. I removed the loop, and the measurements are from Tip of curve to the end of each piece is 54 and 58. The cutting chart says 58.875 83.250. I'm missing something! Ideas? Thanks, Tim
RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Yes, I don't know why the db-201 has such ridiculously long radials, but that is how they were designed. Go figure. I would like to know why though if someone knows... 73 Apologies in advance, this is going to wander a little off-topic and ended up getting long...skip to the bottom if you want the simple answer. The venerable quarter-wave vertical antenna seems like such a simple antenna, but the physics behind it aren't all that simple. It is heavily influenced by the ground (be it created by radials or the actual earth beneath the antenna), and, due to that often being a wide-ranging variable, a lot of myths and ambiguity as far as the right way to create the ground plane or ground system have amassed over the years, especially within the ham radio realm. In its simplest form, a ground-mounted quarter-wave can be thought of as being a dipole with the lower-half of the antenna formed by a ground system, in whole or in part comprised of the earth. You've probably seen crude drawings in antenna handbooks showing the vertical radiator, the virtual mirror image half of the dipole buried below-grade, and imaginary capacitors between the vertical radiator and the ground system to close the circuit, allowing displacement currents to flow back to the feedpoint. That's a decent approximation of how it works when the ground system is actually the earth, and to get a good ground system when the mirror-half of the dipole is actually the earth usually involves installing buried radials to improve the conductivity of the earth to lower losses, thereby improving efficiency. (Sidebar. What are these displacement currents of which you speak? Every circuit, including antennas, require at least two terminals in order for current to flow. You can't force current into a single-terminal device - EE 101. And antennas are no exception. Contrary to popular belief, you can't end-feed an antenna connecting only the center conductor of the coax to the end of the antenna without a ground system or some other way for current to flow back to the shield at the feedpoint. Quite often, there is a a part of the circuit that nobody realizes exists when they claim that they're able to end-feed an antenna, such as a half-wave without a ground, and have it work to some degree. The circuit is closed by stray capacitance between the coax shield and the antenna, even lacking any direct connection to ground or a ground system. The currents set up by the E-field of the antenna have to make their way back to the feedpoint. When these currents flow through a coupled ground return path, rather than being a hard-wired connection such as we would have with an antenna like a folded dipole, these are called displacement currents.) In the idealized world, the feedpoint Z of an infinitely-thin quarterwave over a flat perfectly-conducting ground plane of infinite area is half that of a center-fed dipole in free space, i.e. 36.5 ohms instead of 73 ohms. Matching 36 ohms to a 50 ohm feedline is fairly trivial, but lacking any matching, you're still left with a (roughly) 1.4:1 VSWR, the same as you would have when connecting either a 72 ohm dipole or a 36 ohm quarterwave to 50 ohm line. One simple technique to help improve the match, and often eliminate the need for any external matching network, is to slope the radials downward in order to produce a feedpoint Z somewhere between 36 ohms and 73 ohms, i.e. something closer to 50 ohms. When you take away the earth and synthesize ground by using radials, things change a lot from the theoretical case above. There has been a LOT of emperical research and computer modeling done on the topic, quite a bit even in recent years, regarding the performance (efficiency, Z, pattern, etc.) of antennas with elevated ground radials, most of that research being specific to MW and HF antennas. To grossly over-simplify, in many cases you can get as good, if not better, performance out of a quarterwave with a small number of elevated radials as you can a traditional ground-mounted quarterwave with a lot of buried radials, especially when ground conductivity is less than ideal. At HF, the ground still plays a big role in how these elevated-radial antennas perform. But at VHF and above, with the antenna mounted very high (in terms of wavelength), the earth has much less of an effect. Classic ground-mounted quarterwave antennas (e.g. AM broadcast) use a large number of shallowly-buried radials (typically 120 or more) to create an earth-based ground plane. In this case, the radials don't necessarily need to be resonant. Once you get up in the neighborhood of 120 radials or more, they act less like individual wires and more like one continuous conductive disc. In contrast, in the case of a highly-elevated antenna with a small number (1, 2, 3, 4) of radials, the lengths of the radials are critical; they make the antenna behave more like conventional dipole in regard to their effect on feedpoint
Re: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Various ARRL publications cite the 5% longer rule. Right or wrong, who knows? Chuck WB2EDV - Original Message - I haven't found any factual basis to back up the notion that the radials should be 5% longer than the vertical radiator.
RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Various ARRL publications cite the 5% longer rule. Right or wrong, who knows? Chuck WB2EDV At the risk of offending someone somewhere, the fact that the ARRL cites the 5% rule without backing it up with the analysis behind the rule doesn't really suprise me... Think about it this way. If you made a dipole, would you cut one side 5% longer than the other? Why not make the vertical radiator 5% longer, and leave the radials exactly a quarter-wave, thereby saving on materials? You know I'm not taking a dig at you Chuck - I'm just waiting (hoping) that someone comes up with some real evidence to back up the 5% rule. --- Jeff
Re: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Yep, I know. I just wanted to let you know where the info was coming from. Chuck WB2EDV - Original Message - From: Jeff DePolo j...@broadsci.com To: Repeater-Builder@yahoogroups.com Sent: Sunday, February 21, 2010 4:45 PM Subject: RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? Various ARRL publications cite the 5% longer rule. Right or wrong, who knows? Chuck WB2EDV At the risk of offending someone somewhere, the fact that the ARRL cites the 5% rule without backing it up with the analysis behind the rule doesn't really suprise me... Think about it this way. If you made a dipole, would you cut one side 5% longer than the other? Why not make the vertical radiator 5% longer, and leave the radials exactly a quarter-wave, thereby saving on materials? You know I'm not taking a dig at you Chuck - I'm just waiting (hoping) that someone comes up with some real evidence to back up the 5% rule. --- Jeff
[Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
At the risk of offending someone somewhere, the fact that the ARRL cites the 5% rule without backing it up with the analysis behind the rule doesn't really suprise me... I had personally never seen the ARRL 5% rule printed anywhere that I can remember. Think about it this way. If you made a dipole, would you cut one side 5% longer than the other? Maybe, if you had a reason for offset feed... but most offset Dipoles are not 5% different. I thought the subject was about 1/4 wave ground planes and not dipoles... they are not the same animal. Why not make the vertical radiator 5% longer, and leave the radials exactly a quarter-wave, thereby saving on materials? Because a quarter wave ground plane doesn't work well with a beta match that way. You know I'm not taking a dig at you Chuck - I'm just waiting (hoping) that someone comes up with some real evidence to back up the 5% rule. I'll email you direct... cheers, skipp
[Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
NORM KNAPP nkn...@... wrote: Make the vertical element 50.5 from tip of loop to flat plate where the radials rest.. The radials need to be 73 long... Works for me. 73 In the standard ground plane world... the radials should only need to be about 5% longer than the vertical element. However, longer than 5% is not going to be a huge problem as being less than 5% would be. s. - Original Message - From: Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com To: Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com Sent: Sat Feb 20 16:20:07 2010 Subject: [Repeater-Builder] DB-201 Measurements for 6 Meters? Hi Folks, I looked at the cutting 'chart' here on r-b, but it didn't make much sense. The antennas that I have are at 47.2MHz, and resonate exactly at that frequency. I removed the loop, and the measurements are from Tip of curve to the end of each piece is 54 and 58. The cutting chart says 58.875 83.250. I'm missing something! Ideas? Thanks, Tim
[Repeater-Builder] Re: DB-201 Measurements for 6 Meters?
Thanks Norm Skipp - I'll do some cutting tomorrow. Any clue what the pattern might be hung off the side of a wide spaced tower? (triangular about 15' between legs at the height the antenna will be). Thanks again! Tim W5FN
