Aw shucks, there's no magic involved. I think I figured this out...
Here goes my explanation:
I have to make one assumption for this to work, and that is that noise
arrives at the antenna from all angles roughly equal. Don't know how
true this is, but seems reasonable.
On 160, an inverted vee with apex at 50 ft, ends at 15 ft, is roughly
equivalent to a dipole at 32 ft.
Consider a short dipole, (the beam isn't much different at 160M), it
will have a lot of attenuation but signal and noise will be attenuated
equally, so its actual gain doesn't matter. What does matter is that
the short dipole is 65-32= 33 equivalent feet higher. This will have
approximately 7 dB more gain for the low angle signals om 160M. Assuming
noise arriving at all angles, then there is a 7dB improvement in signal
to noise ratio on the higher antenna.
I was hoping for some magic item I could put to use on other antennas.
If anyone wants to add anything or tear my analysis apart, feel free.
>I have a temporary low inverted vee for 160 meters. The apex is at 50 ft
>ant the ends are at 15 feet. It has a choke on the feedline. Yes, I know
>it's a lousy DX antenna. I also have an XM240, 2 element 40 meter beam
>at 65 feet. The inverted vee is under the beam. On receive on 160 meters
>the 40 meter beam has a much better signal to noise ratio on ALL
>signals. My question is WHY?
>The noise in question is atmospheric noise. I'm not considering TV
>birdies and electric fence noise. The 40 meter beam has a little
>directivity on 160 meters but it is better than the inv vee no matter
>where it is pointed. I looked at the EZNEC response of these antennas to
>see if I could answer the question, but the answer wasn't there, or if
>it was, I sure missed it. The patterns of both of these antennas are
>close to the same. Looking at the patterns, you would expect the
>inverted vee to be little lower noise if the beam was pointed the wrong
>direction, but this isn't the case. The gain on the 40 meter beam is
>about 12 dB below that of the inv vee. That's mainly because of the
>large coax loss due to the huge SWR. And no, the signal to noise ratio
>does not change if I add attenuation when using the inverted vee.
>So, any of you low band noise gurus care to venture a guess why? I
>always had the impression that the only way to improve signal to noise
>ratio on 160 was to narrow the receive lobe pattern. Am I missing
>something? There has to be a logical explaination for this.
>Here is another piece of (maybe unrelated) data. Yesterday I went out
>and quickly put out a 1000 ft BOG (unterminated) for the 160 meter
>test. It had a ground and a common mode choke on the feedline. It
>didn't work. It was noisier than either of the other two antennas.
>Shortened it to 700 ft, and still no good. Didn't use it. This was
>puzzling also, but not as much as the beam/inv vee question.
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