On Tue, 29 Jul 1997 23:27:07 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> To: btippett@CTC.Net, amps@contesting.com
To: <topband@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
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>
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