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dipole length vs db
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dipole length vs db
In article , ml wrote:
If I only have room for a short dipole, say 40 feet, and use a tuner such as the SGC or Icom AH-4, what is the penalty in DB for the lower bands (160, 75, 60, 40) compared to a half wave on each band? 73, Fred K4DII just a fyi, this really belongs in a new thread ML- Perhaps you are right. I was attempting to ask the same question in reverse: The "penalty" is the opposite of the possible improvement. So far I've learned that the majority of loss is in the matching network. Actual gain of a shortened dipole is only about a half dB down, but radiation resistance is so low that I-squared-R losses in the tuner are significant. Someone mentioned that you could reduce losses in the tuner by adding external loading coils. It seems to me that you would just be moving the loss out of the tuner into the loading coils. The question of whether an order of magnitude of improvement can be attained appears to be answered. If you define that as ten dB of improvement, then probably not. The total penalty is most likely less than that. 73, Fred K4DII |
dipole length vs db
On Thu, 12 Oct 2006 23:30:17 -0400, (Fred McKenzie)
wrote: So far I've learned that the majority of loss is in the matching network. Actual gain of a shortened dipole is only about a half dB down, but radiation resistance is so low that I-squared-R losses in the tuner are significant. You are right that the gain of a short practical dipole is good (and that requires that the losses are low). The transmission line is a potential source of high loss when feeding a short dipole. There was an article on QST some time back on using a 66' dipole on all bands. I have written a review which contains some graphs that cast light on where the losses are, the article is at http://www.vk1od.net/LOLL/index.htm . It turns out that for most practical configurations, it is difficult to achieve good overall efficiency when the dipole length is below about 35% of a wavelength. You will probably need parallel wire transmission line rather coax for a shortened dipole. Someone mentioned that you could reduce losses in the tuner by adding external loading coils. It seems to me that you would just be moving the loss out of the tuner into the loading coils. The question of whether an order of magnitude of improvement can be attained appears to be answered. If you define that as ten dB of improvement, then probably not. The total penalty is most likely less than that. Taking an "order of magnitude" to mean a tenfold increase or decrease (10dB), it will be challenging to improve an antenna tenfold by just adding length unless it was very inefficient in the first place (too short, lossy feedline + ATU). If you coax centre fed a 40' dipole on 3.6MHz with 30m of RG58, you would find opportunity for tenfold improvement by increasing length (to a particular value).... but only because it was so hopelessly inefficient as configured. By my 35% suggestion, a typical parallel line centre fed 40' dipole will peform ok down to about 8.6MHz. If it is coax fed, it will only be reasonably efficient on its series resonances (~12MHz, ~36MHz). Owen -- |
dipole length vs db
|
dipole length vs db
ml,
I think the other posters have covered the fact that you just can't get an order of magnitude or two over all bands just by lengthening the wire. Two orders of magnitude, or 20dB gain over a dipole requires really heroic effort in antenna building at HF. If you put a lot of time, money and effort into it, you might be able to get 20dB gain over a dipole on 10 meters. You would have to use stacked long-boom beams or something of that sort. Even 10dB gain over a dipole is going to require a big beam. Think 5 element yagi or so. So, unfortunately, you're not likely to get a gain of more than a few dB over a dipole... maybe 5 or 6dB, like Cecil's antenna on 17m, which is a substantial gain, but falls short of an order of magnitude. Cecil's point is valid. I went a bit overboard on saying that the pattern of a long dipole was BAD. The multiple lobes can be useful if they point in the directions you want to work, it's true. It IS bad if there happens to be a null on a station you want to work. It's good to be able to put the energy you're radiating in the direction you want. For a long multiband antenna you can't rotate, this could be hard because the direction of the peaks and nulls changes with frequency. This could be good, it could be bad, but without pictures, you just don't know which way your signal is going. (you can find some pictures of long doublet patterns here, by the way: http://www.cebik.com/wire/abd.html.) - - - - - - Something to think about regarding massive signal improvement from changing your antenna: Having a high gain antenna means that you get response in the direction you want *at the expense* of other directions. This is why high gain ham antennas are made rotatable. You can send all your transmitted power in a narrow *beam* in the direction you want to work. For a very long doublet, the lobes may have gain over a dipole, and that could certainly be useful for some contacts. It could be, though, that you have a null in the direction you want to work. In the end, that probably all averages out. - - - - - - If the multiple-lobed pattern works for you, just make the antenna 1/2 wavelength long on the lowest frequency of operation. This will give you a big EFFICIENCY boost on the low frequencies, because less of your power will go to heat in the tuner. I wonder, also, how high your antenna is above ground. Putting your antenna higher will improve the signal on all bands, possibly dramatically on the lower frequencies if it's not up at least an eighth to a quarter of a wavelength on the lowest band. You might just try to build something for a single band where you want improvement. That's what I did when I was starting out. I had a 150 foot wire that I used on all bands, but then I started building single band antennas for my favorite bands to improve performance there. 73, Dan |
dipole length vs db
If I only have room for a short dipole, say 40 feet, and use a tuner such
as the SGC or Icom AH-4, what is the penalty in DB for the lower bands (160, 75, 60, 40) compared to a half wave on each band? Some quasi-wild guesses for a 40 foot dipole relative to full size at the same height, probably best case. 160m: -16dB 75m: -8dB 60m: -3dB 40m: -1.5dB These numbers don't mean much. I used EZNEC to look at the losses in Q=100 loading coils with enough reactance to bring thing to resonance on each band. No attempt to match to 50 ohms was made, but I made the Q kind of low to reflect other tuner losses. Still, these are almost certainly optimistic, especially on 160. Dan |
dipole length vs db
On 12 Oct 2006 23:32:20 -0700, "
wrote: If I only have room for a short dipole, say 40 feet, and use a tuner such as the SGC or Icom AH-4, what is the penalty in DB for the lower bands (160, 75, 60, 40) compared to a half wave on each band? Some quasi-wild guesses for a 40 foot dipole relative to full size at the same height, probably best case. 160m: -16dB 75m: -8dB 60m: -3dB 40m: -1.5dB These numbers don't mean much. I used EZNEC to look at the losses in Q=100 loading coils with enough reactance to bring thing to resonance on each band. No attempt to match to 50 ohms was made, but I made the Q kind of low to reflect other tuner losses. Still, these are almost certainly optimistic, especially on 160. Dan, looking at a hypothetical unloaded scenario... I don't know what feedpoint Z you got for a centre fed 40' dipole at 7MHz... but lets guess somewhere about 11+j750. Lets feed the antenna with 15m (~50') of RG58C/U for a loss of 18dB (efficiency 1.6%) and an input z at the ATU of 44-j179. The ATU should handle that with very low loss... so overall, the efficiency is ~1.6%. You can do these calcs with the line loss calculator at http://www.vk1od.net/tl/tllc.php, nothing to unzip! Adding 26' of wire to the dipole should improve the antenna to better than 80% efficiency or a fifty fold improvement on transmit. But it only worked so dramatically because the 40' dipole centre fed with coax is so inefficient. Owen -- |
dipole length vs db
On Fri, 13 Oct 2006 06:51:50 GMT, Owen Duffy wrote:
7MHz... but lets guess somewhere about 11+j750. Lets feed the antenna Spellcheckers aren't smart enough, should read: 7MHz... but lets guess somewhere about 11-j750. Lets feed the antenna -- |
dipole length vs db
Fred McKenzie wrote:
Someone mentioned that you could reduce losses in the tuner by adding external loading coils. It seems to me that you would just be moving the loss out of the tuner into the loading coils. Very little radiation occurs before the tuner. Lots of radiation often occurs between the feedpoint and the loading coils. -- 73, Cecil http://www.w5dxp.com |
dipole length vs db
thanks for the tips appreciate your time in writing the posts :)
my antenna is about 150ft high and rather higher above average terran i am up on a naturall high spot almost a hill the view from the roof is amazing only 1 other building is taller than me for miles i had thought i dunno why that if i ended up w/a dipole that was very long (multi waves) i would get increased gain(significat) at some point but i certainly dont want to end up w/directional gain i wanted a broad freq and broad radation since i like to work all bands and all locations so i learned i still can't have my cake and eat it too stomp i'll prob just follow cecil's and richards' rule, of build it and experiment i'll end up w/a 160m long dipole as it's my lowest freq and see how it goes i'll have to call my tuner manuf as i don't think it will tune that long perhaps i can modify it or i'll have to use a tuner in the shack then i'll just see how it works i guess ok on the lower part but up around 10m if i understand it i'll become more directional an a/b switch and i'll be set i'll have to figure what wire ga will support itself at that length 2 160m runs is big i can support it in the middle on one side but the other is free thanks very much i really did learn alot m In article . com, " wrote: ml, I think the other posters have covered the fact that you just can't get an order of magnitude or two over all bands just by lengthening the wire. Two orders of magnitude, or 20dB gain over a dipole requires really heroic effort in antenna building at HF. If you put a lot of time, money and effort into it, you might be able to get 20dB gain over a dipole on 10 meters. You would have to use stacked long-boom beams or something of that sort. Even 10dB gain over a dipole is going to require a big beam. Think 5 element yagi or so. So, unfortunately, you're not likely to get a gain of more than a few dB over a dipole... maybe 5 or 6dB, like Cecil's antenna on 17m, which is a substantial gain, but falls short of an order of magnitude. Cecil's point is valid. I went a bit overboard on saying that the pattern of a long dipole was BAD. The multiple lobes can be useful if they point in the directions you want to work, it's true. It IS bad if there happens to be a null on a station you want to work. It's good to be able to put the energy you're radiating in the direction you want. For a long multiband antenna you can't rotate, this could be hard because the direction of the peaks and nulls changes with frequency. This could be good, it could be bad, but without pictures, you just don't know which way your signal is going. (you can find some pictures of long doublet patterns here, by the way: http://www.cebik.com/wire/abd.html.) - - - - - - Something to think about regarding massive signal improvement from changing your antenna: Having a high gain antenna means that you get response in the direction you want *at the expense* of other directions. This is why high gain ham antennas are made rotatable. You can send all your transmitted power in a narrow *beam* in the direction you want to work. For a very long doublet, the lobes may have gain over a dipole, and that could certainly be useful for some contacts. It could be, though, that you have a null in the direction you want to work. In the end, that probably all averages out. - - - - - - If the multiple-lobed pattern works for you, just make the antenna 1/2 wavelength long on the lowest frequency of operation. This will give you a big EFFICIENCY boost on the low frequencies, because less of your power will go to heat in the tuner. I wonder, also, how high your antenna is above ground. Putting your antenna higher will improve the signal on all bands, possibly dramatically on the lower frequencies if it's not up at least an eighth to a quarter of a wavelength on the lowest band. You might just try to build something for a single band where you want improvement. That's what I did when I was starting out. I had a 150 foot wire that I used on all bands, but then I started building single band antennas for my favorite bands to improve performance there. 73, Dan |
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