Good points Tom, thanks.
Just one comment regarding paragraph 2 where you say that there is a
'filter' that often has more attenuation of horizontally polarized signals
than vertically polarized.
I have found this to be totally the opposite. I do a lot of EME and on both
144 and 432 MHz and with my systems (homebrew x-pol yagis for 144 and a
28'dish with dual dipole feed for 432) I can instantly switch polarization
from horizontal to vertical. I certainly see that the surrounding objects
attenuate vertical more than horizontal. There is quite a big difference
actually, easily detectable on returning moon echoes. In different
directions I have objects in the pattern both in the near field and in the
far field, similar situation.
Could be different on HF, but to me it is not obvious why that would be.
73/Peter SM2CEW
www.qsl.net/sm2cew
At 12:36 2004-06-26 , you wrote:
>> At 03:44 AM 6/26/2004, Peter Sundberg wrote:
>> >So f/b on groundwave is important and so far, from what I
>have seen the
>> >results are pretty close to what the software model tells
>you. >
>>
>> My recent experience leads me to agree, Peter.
>
>> I understand that anything near the antenna can and will
>affect groundwave
>> field strength, but since the only variables in this test
>were the pointing
>> of the antenna and the frequency, this result seems to be
>a pretty good
>> indication that the reflector is tuned too high.
>
>Something either is a good method, or it isn't. Not being
>good doesn't mean every case won't work nor does it mean
>most cases won't work, this method simply has the potential
>for anything up to a very large error and you have no way to
>know if it working or not without using another more
>reliable method to crosscheck.
>
>If you are fortunate enough to have an uncluttered area, NOT
>just in the path but in all other directions around the
>antenna, and if the antennas at both ends have very low
>response to vertically polarized signals then you probably
>can use longer distances to indicate zero angle gain and F/B
>ratio. But we have to remember what we are measuring:
>
>1.) We would be measuring the pattern at zero degrees,
>something we don't often care about.
>
>2.) We would be measuring through a "filter" that often has
>significantly more attenuation for horizontally polarized
>signals than vertically polarized signals.
>
>3.) The most reliable way to measure gain or F/B of a
>horizontally polarized HF antenna is with spacing in the
>order of hundreds of feet, not thousands or more. The
>"measure at a mile" stuff came right out of the thin air,
>and became a myth. People actually think it is better when
>it is very much worse!
>
>The error probably is modest (a few dB) in most cases when
>sites are reasonable clear of sources of re-radiation at
>both ends (in *every* direction from the antennas) and the
>antennas have minimal vertically polarized zero degree
>radiation. The main point is the data is unreliable. There
>CAN be a significant error, like the example I cited where a
>person became totally convinced he improved antenna
>efficiency dramatically by changing only one thing, his
>matching device, from a tuner to a stub. There is at least
>one commercial Yagi and one small Quad being sold that use
>totally sloppy methods to claim more gain than exists in
>theory. You can bet the gain is 2-3dB less than claimed, and
>is actually in the same ballpark as other antennas the same
>size.
>
>I've even seen one person claim, with a straight face, his
>special design quagi beam had five or ten dB gain over a
>Hygain 204BA! Pure nonsense, but you'll never convince the
>person who has managed to get the results he wants through a
>flawed test that he might be mistaken.
>
>A lot of nonsense comes from using unreliable measurement
>techniques and not cross-checking the results. A measurement
>is either reliable, or it isn't.
>
>73 Tom
>
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