TopBand: 160m shunt feeds

[km1h @ juno.com]

On Tue, 29 Jul 1997 23:27:07 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> To:            btippett@CTC.Net, amps@contesting.com
>> Date:          Wed, 30 Jul 97 01:11:20 +0000
>
>> Although it obviously radiates Bill, I question the efficiency of a 
>#12
>> wire 4' away from a large tower cross section. 
>> Do you have anything to show that this is a correct  d1/d2 ratio and
>> spacing?  It just does not seem proper IMO and I would think that it
>> could be improved. 
>
>Hi Carl,
>
>While a larger conductor would have less loss, the loss is so low to 
>begin with you could never notice the change.
>
>Loss is only due to resistive losses, reactances do not cause loss.  
>There would be very little loss difference if the wire was #16 or 
>four inch copper pipe, since the current is so low  and resistance so 
>low even in the copper wire.

Perhaps I used the wrong word but we mostly agree here Tom. My concern
was indeed with bandwidth and the High Q, narrow bandwidth, high voltage
that a single #12 would bring. The #12 was my first attempt back in 1984.


You can probably help my memory here, but when I was running FS tests the
larger I went in diameter, the better the signal at the remote site. We
used a BC professional meter (Granger rings a bell) that covered from
roughly 500KHz to 2.5 MHz in a field 2.5 miles away. 
The tests were run for many hours over 2 days with me doing the tower
climbing. I tried spacings of 8 to 36" in 4" increments  and found that
12-16" gave the best FS with both the #12 and the .75" .  The FS dropped
rapidly over 24"; this was possibly due to the FS site being directly
behind the tower/feed sight line and the feed beginning to actually
radiate.  

When my 1986 article was published I had calls and correspondence with
others that reported marked directivity with the #12 at spacings in the
3-4' range.  Later on I found that John True preferred 10" spacing and 1"
diameter. Others reported superior (their words) with 2" irrigation
tubing as the gamma rod.

Since most literature and commercial yagis use a 4:1 or less D1/D2 ratio
AND that most commercial yagis in the 7-50MHz range use  2.5 to 5"
spacing I fail to understand why some 160M feeds are so far from the
norm.  IMHO opinion efficiency/gain-bandwidth products, or whatever you
prefer to call them, has to suffer. 

Maybe we are not talking big numbers here, in dB, but every little bit
helps on topband. I would still be using a shunt fed tower but when I
moved I swore to never go thru the hassle of 25000 feet of radials again!

This could be the beginning of another topic, but what is the ideal # of
elevated radials? Use 15-20' elevation as an example. The original
article in QST said one to four.  I started with 4 and now am up to 12
based on comments from K1ZM and others. 


>
>It does make the antenna much broader however, since the thicker 
>gamma adds less inductive reactance so the series tuned feed system 
>has lower Q.
>
>> An accurate way to grid dip a tower was explained in my 12/86 CQ 
>article.
>> Simply drop a #12 or so wire from the very top of the tower and 
>couple
>> loosely to the gdo at the base via a one turn loop and the antenna
>> appears as a folded unipole.  This required a minimum of 4 radials 
>at the
>> tower  to function properly for me. .
>
>That does not give an accurate measurement of self-resonant 
>frequency, because the gamma wire forms a stub with the 
>tower, and acts like a transmission line.

Only when there are no radials was my experience. In a folded monopole
the only result is a step up in impedence which made loose GDO coupling
simpler.  The radials removed the stub effect .


>
>I have an insulated 135 ft tower with a top hat that used to be 
>grounded. I just modified it two weeks ago. I shunt fed that tower 
>as part of a two element phased array, and I have the drop wires 
>still hanging down the side. 
>
>I measured the resonant frequency by using one drop wire and a GDO.
>With the tower base grounded, I measured about 1300 KHz. 
>
>When I tied the drop wire to the tower and measured the resonant 
>frequency with the GDO coupled to a link between the tower base and 
>ground, it measures about 1600 KHz.    


What did "ground" consist of ?


>But who cares? All you really need to know is the impedance of the 
>feedpoint. If the tower is resonant below 160, all it does is reduce 
>the bandwidth slightly. The only horror story would be if the tower 
>approached or went below 900-1000 KHz.  

Yep, anything much over 3/8 wave only keeps the worms warm on 160M. 
 

>In that case an electrically fat skirt or cage surrounding the tower 
>could be used to raise the self-resonant frequency anyway.
>
>73, Tom W8JI 
>


73....Carl   KM1H
>FAQ on WWW:               http://www.contesting.com/ampfaq.html
>Submissions:              amps@contesting.com
>Administrative requests:  amps-REQUEST@contesting.com
>Problems:                 owner-amps@contesting.com
>Search:                   http://www.contesting.com/km9p/search.htm
>

--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com