[TowerTalk] Actual LP Performance vs Tribanders

[Jim Smith]

Hmm... This would appear to have severe ramifications for tuning the 40m 
portion of my C4SXL.  F12 has a tuning procedure on their web site which 
involves adjusting the jumpers on the linear loading wires to achieve 
best F/B and then adjusting the hairpin for best SWR.  I live on a 33 x 
120 ft city lot.  The C4 will be up about 50 ft which handily clears all 
the nearby roof tops but not the trees lining the streets or the HV 
power line running down the lane at 50 ft.  I was planning on asking a 
nearby ham (a mile or two) to provide a signal for tuning purposes.  
There's all kinds of clutter between him and me, he's about 200 ft lower 
in elevation than me and his antenna is quite low.  Sounds like this 
isn't a good idea.

Maybe I could take my HP3400A AC VTVM (good up to 10 MHz or so - should 
be fine for 40m relative measurements) with a small (tuned?) loop a 
block or two away and use it as a field strength meter.  The elevation 
would be about 30-40 ft below the C4.  Line of sight other than some 
tree foliage. 

Could do the same with the 706, I guess, but would be giving up a lot in 
relative FS measurement accuracy.  Then again, I could connect the 3400A 
across the Tx feed line, adjust the Tx output for constant S meter 
reading on the 706 as the beam is rotated (a la N4ZR) and note the 3400A 
reading.  Presuming that the MkV minimum output is 1W then I'd have a 23 
dB range of adjustment.  More than enough for this purpose.

Another possibility would be to enlist the help of a ham who lives on 
the side of a mountain across a body of water from me.  I think his 
elevation might be higher than mine by 100-200 ft.  I'd guess he's about 
10 miles away, certainly more than 5.

Another possibility would be to use a GDO to set the resonant 
frequencies of the DE and reflector and then just adjust the hairpin for 
best SWR.  Anybody know what those frequencies would be for a contester 
who wants to operate cw, phone and RTTY and has the 40m relay box for 
phone operation?  This requires knowing the desired DE resonant 
frequency for when the relay is both energized (cw band) and 
de-energized (phone band).  I gather that I would want the reflector 
tuned to a frequency below 7.000 so it doesn't turn into a reflector 
when I'm at 7.001.

I've got an Autek RF1.  Could I use that somehow instead of the GDO?  I 
do know that min SWR does not necessarily indicate the resonant 
frequency so tuning the elements for min SWR referred to 50 ohms clearly 
won't work.

I would be very appreciative of any comments or suggestions as to how to 
proceed with tuning the C4SXL on 40, given the constraints of my situation.

Sorry for the staleness of this.  Haven't had time to keep up with this 
reflector for some time.  Getting there, though, only 1,050 more posts 
to read (not counting other reflectors).

73 de Jim Smith   VE7FO



Tom Rauch 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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>
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