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An antenna question--43 ft vertical
On Tuesday, June 30, 2015 at 2:36:51 PM UTC-4, rickman wrote:
On 6/30/2015 12:40 PM, Tom W3TDH wrote: I know that what I am about to say is provocative to some but I still think it is worth saying. If you look at the way that commercial and military radios are matched to antennas you will notice that most of the matching is done as close to the feed point as practical. Since only the power that actually reaches the antenna can be radiated I have a hard time seeing the point of matching the transmitter to the feed line. Matching at the feed line connection point will prevent damage to the transmitter but if that were the main objective a dummy load would accomplish that. When you couple the antenna to the load at the feed point you can have extremely low losses in the feed line. When you do the matching at the feed point you will transfer the most energy possible to the antenna and will get the highest available effective radiated power. Since the objective is the transfer of the highest practical amount of power to the antenna the place to do that is at the feed point were possible. I do realize that it is often simpler and easier to match at the feed line connection but I felt obliged to point out that is is not the most effective place to do the job. Has it occurred to you that it might be important to match impedance both at the transmitter and at the antenna? When the feed line is not impedance matched to the transmitter output the maximum power is not transferred into the feed line. Then you have already lost power that can't be recovered by the matching at the antenna even if it is perfect. Your statements are not really provocative, they are just incomplete and/or wrong. -- Rick Rick OK I'll buy incomplete and therefore wrong. Now given a Fifty Ohm feed line connected to a transmitter that is designed for that impedance at the antenna connector does not the actual mismatch occur at the antenna feed point? Certainly that can be compensated for at the transmitter but isn't there a likelihood or at least a risk that you will loose significant effective radiated power in spite of adjusting the apparent feed line impedance to the transmitter? If I do the matching at the feed point will I not maximize the effective radiated power of the antenna by installing the tuner at the feed point. I have already conceded that it is not as convenient to do the matching at the feed point. I do not allege that doing the matching at the transmitter end of the feed line is inherently ineffective only that there is a greater likelihood of loosing ERP needlessly and invisibly if the matching is done at transmitter end of the feed line. By this I mean to ask if I may well deceive the power meeter into showing more power out then I am actually getting. If any power lost is very likely to be insignificant at a practical level than help me to understand why that would be true and I will sell off my Icon AH-4, together with the control converter that allows my Yaesu FT-857D to control it, and my SGC SG-235 and go back to using the Yaesu FC-30 tuner with my FT-857D and the built in tuner on my Yaesu FT-1000. This is especially important for me to get right with my FT-857D since it is the transceiver that I use for my personal go kit. If putting the Icon AH-4 on the mast and running the control line in addition to the coaxial cable is a waste of time I would really appreciate knowing that. Thank you for helping with my education on this issue. -- Tom Horne W3TDH |
An antenna question--43 ft vertical
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An antenna question--43 ft vertical
On 6/29/2015 3:47 PM, Wayne wrote:
"John S" wrote in message ... On 6/29/2015 10:48 AM, Wayne wrote: As a lead in, I use a 16 ft vertical on 20-10 meters, mounted on a flat metal roof. The antenna is fed with about 25 feet of RG-8, and there is a tuner at the transmit end. You use a 16ft vertical as a lead-in? For what and how is that done? Grammatically, the description of the vertical is a lead in for the question, not an actual antenna lead. What are the dimensions of the metal roof? Somewhat irrelevant to my question. But it's about 20 by 35 feet. I'm not looking for an analysis of the existing antenna. While I'm pretty happy with the antenna, I'd like to simplify the matching. To what matching do you refer? You don't want to use the tuner, or is there some other stuff you have not mentioned? I want the tuner matching to be less awkward on some bands. I'm willing to live with the existing high SWRs on the upper bands. Thus, the question: what is the purpose of a 1:4 unun on a 43 foot vertical? ( I assume the "4" side is on the antenna side.) You wrote that you were interested in a 16ft vertical. Now it is a 43ft vertical? Please disregard all about the 16 ft vertical. I'm asking about a 43 ft vertical 1:4 unun. I'd expect a better coax to antenna match when the antenna feedpoint is a high Z (example, at 30 meters), but I'd also expect a worse coax to antenna match when the feedpoint is a low Z (example, at 10 meters). Is that the way it works, or is there other magic involved? All this depends on your answers to the above questions. So, lets begin again, with no distractions. What is the purpose (or benefit) of using a 1:4 unun on a 43 ft vertical. Ok. Well, 43ft is a half wavelength at about 12MHz. The vertical will be very high impedance at that frequency and a 1:4 unun will theoretically bring that impedance down closer to the feed line impedance. Does this help? |
An antenna question--43 ft vertical
In message , John S
writes On 6/29/2015 3:47 PM, Wayne wrote: "John S" wrote in message ... On 6/29/2015 10:48 AM, Wayne wrote: As a lead in, I use a 16 ft vertical on 20-10 meters, mounted on a flat metal roof. The antenna is fed with about 25 feet of RG-8, and there is a tuner at the transmit end. You use a 16ft vertical as a lead-in? For what and how is that done? Grammatically, the description of the vertical is a lead in for the question, not an actual antenna lead. What are the dimensions of the metal roof? Somewhat irrelevant to my question. But it's about 20 by 35 feet. I'm not looking for an analysis of the existing antenna. While I'm pretty happy with the antenna, I'd like to simplify the matching. To what matching do you refer? You don't want to use the tuner, or is there some other stuff you have not mentioned? I want the tuner matching to be less awkward on some bands. I'm willing to live with the existing high SWRs on the upper bands. Thus, the question: what is the purpose of a 1:4 unun on a 43 foot vertical? ( I assume the "4" side is on the antenna side.) You wrote that you were interested in a 16ft vertical. Now it is a 43ft vertical? Please disregard all about the 16 ft vertical. I'm asking about a 43 ft vertical 1:4 unun. I'd expect a better coax to antenna match when the antenna feedpoint is a high Z (example, at 30 meters), but I'd also expect a worse coax to antenna match when the feedpoint is a low Z (example, at 10 meters). Is that the way it works, or is there other magic involved? All this depends on your answers to the above questions. So, lets begin again, with no distractions. What is the purpose (or benefit) of using a 1:4 unun on a 43 ft vertical. Ok. Well, 43ft is a half wavelength at about 12MHz. The vertical will be very high impedance at that frequency and a 1:4 unun will theoretically bring that impedance down closer to the feed line impedance. Does this help? It was been pointed out to me that the figures for feeder loss with an imperfect SWR are only correct when the length is fairly long (at least an electrical wavelength?). How much loss does 25' of RG-8 really have at 12MHz, when there's a halfwave hanging on the far end? -- Ian |
An antenna question--43 ft vertical
On Tue, 30 Jun 2015 15:13:55 -0400, Jerry Stuckle
wrote: Yes, it's most effective to match the feedline to the antenna at the antenna connection. But it's also important to match the transmitter to the feedline. This latter piece is often ignored because people will use a feedline who's characteristic impedance matches the transmitter already (i.e. 50 ohm line for a 50 ohm transmitter). However, there are exceptions. For instance, if you're feeding a 75 ohm antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna will provide a good match (ideally, 1:1). But there will be a 1.5:1 mismatch to a 50 ohm transmitter. In this case it would be better to have the matching network at the transmitter. We may have had this discussion before. Matching a 75 ohm load to a 50 ohm source might be academically interesting, but the actual loss is almost negligible. for a VSWR of 1.5, the return loss is 14dB and the load mismatch attenuation is 0.177dB. That's about what I would expect to lose in two coax connector pairs. You could also feed the antenna with 50 ohm feedline and place the matching network at the antenna. The effect would still be a 1:1 SWR, but the lower impedance of the coax would create higher i^2R losses; not important if you're talking a short line, but a longer one would lower output at the antenna. True, but for roughly equivalent sizes of coax cables, the 75 ohm cable has less loss and the equivalent 50 ohm cable. If you want to handle high power, use 50 ohms. If you want low loss, use 75 ohms: http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm Note that these are for air dielectric cables. Things are not so neat if we consider the dielectric. See the bottom paragraph and graphs: http://www.microwaves101.com/encyclopedias/why-fifty-ohms Dielectric Dielectric const Minimum loss impedance solid PTFE 2.2 50 ohms foam PTFE 1.43 60 air 1.0 75 RG-6/u CATV 75 ohm foam coax still has slightly less loss than the equivalent 50 ohm cable, but not as much as I've previously claimed. This is cute: http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/ "A good rule of thumb is that if the device being connected via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal." -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
An antenna question--43 ft vertical
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An antenna question--43 ft vertical
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An antenna question--43 ft vertical
"Dave Platt" wrote in message ... In article , Wayne wrote: On the 43 foot vertical it could be that the 4:1 unun provides a lower amount of SWR induced additional feedline loss on high Z feedpoints. Of course, the unun might increase the SWR induced additional feedline loss for smaller Z. From the charts, that kinda does appear to be the case. Add into consideration the fact that losses go up with the square of the current. Another issue is the other aspect of SWR - voltage. If you're trying to run "legal limit" or close to it, high SWR on the feedline coax could exceed the voltage rating of the coax dielectric, and you'd get arcing in the coax (or arcing at the connector between your feedline and shack tuner). Installing a hefty 4:1 unun right at the antenna would reduce the worse-case voltage on the feedline, and inside the shack tuner considerably. Depending on feedline length, that might be even more of a consideration than the increase in peak feedline current and the associated I^2*R losses. Good points. I'm running low powers at the moment, but there was a time where I burned up a lot of stuff running a KW :) |
An antenna question--43 ft vertical
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An antenna question--43 ft vertical
"Jeff" wrote in message ... If there is a mismatch at the antenna (and there is no matching at the antenna), then maximum power transfer will occur when the conjugate match is applied at the transmitter end of the feedline. Surely a conjugate match will only match the load if the coax length is 1/2 wavelength or multiple thereof, and the feeder is also lossless. Any other coax length will introduce a phase shift that will require a different match. Yes, I'm assuming that the antenna tuner conjugate match is for the end of the feedline, not for the antenna itself. Wayne W5GIE exiled to W6 :) |
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