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An antenna question--43 ft vertical
In article ,
Wayne wrote: So, lets begin again, with no distractions. What is the purpose (or benefit) of using a 1:4 unun on a 43 ft vertical. http://www.eham.net/articles/21272 has a nice analysis. It looks to me as if: - Without a 4:1 unun, the antenna provides a very nice match at three frequencies with in the HF band. At other frequencies, the SWR is up over 10:1 much of the time - high enough that a coaxial feed can be rather lossy. - With a 4:1 unun, you do lose the excellent match at those three frequencies... but the match gets better at most other frequencies. The SWR across the HF band is much more uniform, and lower on average... low enough to cut the coax losses somewhat and (I think) within the matching range of many rigs' "line flattener" built-in autotuners. |
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An antenna question--43 ft vertical
"Dave Platt" wrote in message ... In article , Wayne wrote: So, lets begin again, with no distractions. What is the purpose (or benefit) of using a 1:4 unun on a 43 ft vertical. # http://www.eham.net/articles/21272 has a nice analysis. # It looks to me as if: # - Without a 4:1 unun, the antenna provides a very nice match at three # frequencies with in the HF band. At other frequencies, the SWR is # up over 10:1 much of the time - high enough that a coaxial feed # can be rather lossy. # - With a 4:1 unun, you do lose the excellent match at those three # frequencies... but the match gets better at most other # frequencies. The SWR across the HF band is much more uniform, and # lower on average... low enough to cut the coax losses somewhat and # (I think) within the matching range of many rigs' "line flattener" # built-in autotuners. Thanks Dave. I'll have to spend some more time studying it, but the article is along the lines of what I was looking for. I would assume that the 1:4 causes behavior just as you say....worse SWR at nearly matched frequencies and better SWR elsewhere. I'll have to pull out some textbooks and see how the math works out for a Z seen through a 1:4 unun. In practice, I've had good results with SWRs even in the 30:1 range with short coax feeds. More research...and thanks. |
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An antenna question--43 ft vertical
On Monday, June 29, 2015 at 8:46:47 PM UTC-4, Wayne wrote:
"Dave Platt" wrote in message ... In article , Wayne wrote: So, lets begin again, with no distractions. What is the purpose (or benefit) of using a 1:4 unun on a 43 ft vertical. # http://www.eham.net/articles/21272 has a nice analysis. # It looks to me as if: # - Without a 4:1 unun, the antenna provides a very nice match at three # frequencies with in the HF band. At other frequencies, the SWR is # up over 10:1 much of the time - high enough that a coaxial feed # can be rather lossy. # - With a 4:1 unun, you do lose the excellent match at those three # frequencies... but the match gets better at most other # frequencies. The SWR across the HF band is much more uniform, and # lower on average... low enough to cut the coax losses somewhat and # (I think) within the matching range of many rigs' "line flattener" # built-in autotuners. Thanks Dave. I'll have to spend some more time studying it, but the article is along the lines of what I was looking for. I would assume that the 1:4 causes behavior just as you say....worse SWR at nearly matched frequencies and better SWR elsewhere. I'll have to pull out some textbooks and see how the math works out for a Z seen through a 1:4 unun. In practice, I've had good results with SWRs even in the 30:1 range with short coax feeds. More research...and thanks. 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. Tom |
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An antenna question--43 ft vertical
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 |
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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 |
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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
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An antenna question--43 ft vertical
On 6/30/2015 12:40 PM, Tom W3TDH wrote:
On Monday, June 29, 2015 at 8:46:47 PM UTC-4, Wayne wrote: "Dave Platt" wrote in message ... In article , Wayne wrote: So, lets begin again, with no distractions. What is the purpose (or benefit) of using a 1:4 unun on a 43 ft vertical. # http://www.eham.net/articles/21272 has a nice analysis. # It looks to me as if: # - Without a 4:1 unun, the antenna provides a very nice match at three # frequencies with in the HF band. At other frequencies, the SWR is # up over 10:1 much of the time - high enough that a coaxial feed # can be rather lossy. # - With a 4:1 unun, you do lose the excellent match at those three # frequencies... but the match gets better at most other # frequencies. The SWR across the HF band is much more uniform, and # lower on average... low enough to cut the coax losses somewhat and # (I think) within the matching range of many rigs' "line flattener" # built-in autotuners. Thanks Dave. I'll have to spend some more time studying it, but the article is along the lines of what I was looking for. I would assume that the 1:4 causes behavior just as you say....worse SWR at nearly matched frequencies and better SWR elsewhere. I'll have to pull out some textbooks and see how the math works out for a Z seen through a 1:4 unun. In practice, I've had good results with SWRs even in the 30:1 range with short coax feeds. More research...and thanks. 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. Tom Tom, very close. 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. 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. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
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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 |
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