really long control cable on a screwdriver
 

I'm using a KJ7U,www.kj7u.com, screwdriver antenna and its "control cable",
for lack of a better term, is around 60 feet long. At this length it
seems to be causing tuneing problems.

It appears that the cable is trying to radiate and thus it causes the SWR
to rise above what it should be. The best way I can validate this is to
tune it the best I can then go unplug the cable from the antenna. When I
do this the SWR drops down to around 1.1 or so. Otherwise on some bands it
is as high as 3 - 4

I put some toriod? cores on the line which helped out the problem on 80m
to some extent. It did help bring the swr back down to a more acceptable
level, but it still isn't as low as it is when the cable is unplugged.

When I had this antenna mounted in a different location and the cable was
considerable shorter, none of this seemed to happen.

Anyone have any ideas or experiences with something like this ?



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Registered Linux User #346565
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WA5K

Jeremy,
Hmm, sounds 'reasonable' to me. A 60' control cable
is about 1/4 wave on 80 meters, and if you're using the
antenna on 80 meters, well...?
'Doc

PS - Sort'a suprised it hasn't singed your mustache, LOL.

I put some toriod? cores on the line which helped out the problem on 80m
to some extent. It did help bring the swr back down to a more acceptable
level, but it still isn't as low as it is when the cable is unplugged.

Keep adding some more and at different distance too. Also balun at the
feedpoint would help and if possible rerouting the cables.

Yuri, K3BU.us

Maybe use some capacitors from the control wires to ground to short out the
rf right to ground before it gets to the toroids. The toroids then won't
have so much rf to work with and they may not be as likely to saturate.

Be careful with the capacitors that you get transmitting style that can
handle a good amount of rf current.

Yuri Blanarovich wrote:


I put some toriod? cores on the line which helped out the problem on 80m
to some extent. It did help bring the swr back down to a more
acceptable
level, but it still isn't as low as it is when the cable is unplugged.

Keep adding some more and at different distance too. Also balun at the
feedpoint would help and if possible rerouting the cables.

Yuri, K3BU.us

I'll try putting them at different locations along the cable. I have 4 that
are designed to clip onto wire the size of rg-8u so I coiled up the
control cable and cliped the cores over the coil of wire, and i put all 4
on that coil. I'll try making several smaller coils and putting them at
different places along the cable


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http://www.tblx.net
WA5K

Allan Butler wrote:

Maybe use some capacitors from the control wires to ground to short out
the
rf right to ground before it gets to the toroids. The toroids then won't
have so much rf to work with and they may not be as likely to saturate.

Be careful with the capacitors that you get transmitting style that can
handle a good amount of rf current.

I've heard of doing this before, with the capacitors, so would this be the
ceramic type? and how would i distinguish between "transmitting" style and
regular capacitors?



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Registered Linux User #346565
http://www.tblx.net
WA5K

Jeremy Salch wrote:
I've heard of doing this before, with the capacitors, so would this be the
ceramic type? and how would i distinguish between "transmitting" style and
regular capacitors?

Doorknobs are excellent but expensive.
--
73, Cecil http://www.qsl.net/w5dxp



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On Wed, 28 Apr 2004 13:06:59 -0500, Jeremy Salch
wrote:

Allan Butler wrote:

Maybe use some capacitors from the control wires to ground to short out
the
rf right to ground before it gets to the toroids. The toroids then won't
have so much rf to work with and they may not be as likely to saturate.

Be careful with the capacitors that you get transmitting style that can
handle a good amount of rf current.

I've heard of doing this before, with the capacitors, so would this be the
ceramic type? and how would i distinguish between "transmitting" style and
regular capacitors?

Hi Jeremy,

There should be no distinction between caps for this job - almost
anything will work as the control voltages are so low that even
dielectric breakdown is unlikely.

On the other hand, the package style may be more carefully considered.
The high end designs use what are called "feed-thru" button styles
that are designed for bulkhead connections with a pig-tail on either
side and the cap looks like a threaded stud with a collar. These are
specifically designed to decouple rf off of dc lines. They provide
the shortest lowest inductance path to ground (essentially zero lead
length).

These would be a must for UHF, but at HF you are just as well off with
simple, leaded Micas, Ceramics, Paper-oil, polystyrene, tantalum, or
any of a host of others. You don't really want massive capacitance
(which is to say perhaps tantalums and like are unnecessary overkill)
because 10,000pF is more than enough but 0.22mF works just as well.
If you assist the cap with a small choke (on the noisy side of the
current) you gain that much more isolation. Again, any value is
better than none (but it hardly begs 1H audio chokes).

73's
Richard Clark, KB7QHC

Cecil Moore wrote:

Jeremy Salch wrote:
I've heard of doing this before, with the capacitors, so would this be
the
ceramic type? and how would i distinguish between "transmitting" style
and
regular capacitors?

Doorknobs are excellent but expensive.
--
73, Cecil http://www.qsl.net/w5dxp



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is that a kind of capacitor, or are you making fun of my not knowing the
different kids of capacitors


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WA5K

Jeremy Salch wrote:

Cecil Moore wrote:

Jeremy Salch wrote:
I've heard of doing this before, with the capacitors, so would this be
the
ceramic type? and how would i distinguish between "transmitting" style
and
regular capacitors?

Doorknobs are excellent but expensive.
--
73, Cecil http://www.qsl.net/w5dxp



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is that a kind of capacitor, or are you making fun of my not knowing the
different kids of capacitors



No he is not making fun of you. There is a style of capacitor that is
called a door knob for the shape that it is made in.

To start out with go ahead and try some good high voltage ceramic capacitors
from the control lines to ground. A fairly high value of 0.47 uF might do
good in parallel with a .01 uF cap.

For the toroids in the line they have to be fairly hefty as there could be
some significant currents running on the lines at the different frequencies.
If the toroids saturate they won't work as intended and can cause some real
interesting things to happen.