To: <towertalk@contesting.com>
>Date: Fri, 16 Oct 1998 03:21:37 +0100
>From: Mike <no6x@inreach.com>
>
>Well, I am at it again, trying to re-invent the wheel. In
>looking through some old qst's last night (cable was out) I found
>an article about a 160 meter half sloper that used inductive
>loading at the feeedpoint. I know this is not the best way to do
>it but, If I can make this work then I know I can do it and may
>venture into other things.
If the sloper is attached to the correct point on the tower, very
little radiation takes place from the sloper wire itself. So
inductive loading to shorten this wire is not particularly
detrimental to efficiency unless the inductor is quite lossy.
Bottom line is that this is not a bad thing to consider.
>Now the gentleman in the article used 90 turns on a 1 inch form
>of fairly small wire, and was using a 40 foot tower for the
>other half. I tried to guess and did not even come close,
>please read on. This is for a friend of mine's place that
>wanted something more on 160. He has a 60 foot tower and fairly
>limited space.
A lot depends on the exact nature of what he has on top of the
tower. There is also some variation possible depending on how
the tower is grounded and how the feedlines (and rotor control
lines, etc.) are dressed on the tower and brought away from it.
This is why there has been so much variability in the results of
hams trying to make half slopers work. Some do very well indeed,
and some conclude that they aren't possible to make work.
A great deal of cut and try can be avoided by modeling the tower
system as accurately as possible before beginning the outside
work. That is probably the best advice that can be given to
anyone starting out to put up a half sloper on a tower.
Fortunately, you are working at 160 meters on a tower short
enough that you are unlikely to run into any of the really
pathological conditions that frequently kill 80 meter slopers.
If you can model or get someone to model the system, it will be
easy to identify the optimum point on the tower for the feedpoint
attachment. The value of the loading inductor will be predicted
to within 5 to 10 percent of the correct value. If you want the
thing to match 50 ohms directly, you will know the value of the
shunt matching inductor required for this to within 10 percent or
so before touching a soldering iron. You can spend your outdoor
work time pruning things to perfect the match and put the center
frequency where you want it rather than trying to figure out why
the thing doesn't seem to be an antenna.
>Here is what I did. I wrapped 41 turns of #14 wire on a 3 1/2
>inch form, then ran about 80 feet of wire from the other end of
>the coil. Net return: ZILCH It did not even come close to 160
>meters, we did find it to be a fairly good recieve antenna on
>1.2 MHZ though.
>
>Now what is it that I need to change, or better yet, can someone
>point me to something (formula, web page, etc.) that will allow
>me to do it correctly the second time.
Nobody can point you to exact simple calculations to even get you
into the ball park because of all the uncontrolled variables
associated with the tower system.
>Another friend of mine has the specs to build a base loaded
>antenna with a 8ft.whip(for mobile use). Would things be
>proportional. i.e. if it uses an 8 foot whip and 135 turns on a
>3 inch form could I use an 80 foot "whip" and 13.5 turns?
>
>Please help.
>The dumb kid in W6
>Mike...NO6X
Unfortunately, things don't scale nicely like this.
If you are pretty sure that your feedpoint is in the right place
(But without modeling, how would you know?), then you could
proceed with a grid dip oscillator to find the system resonant
frequency. Then adjust the inductor accordingly.
73, Eric N7CL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|