|[AMRadio] antenna tuners|
w1eof at hamnutz.com
Thu Apr 20 15:51:02 EDT 2006
Don & All,
I'm about to dig a trench in my yard to run some coax and control cables to
the back, in the bushes, our of harms (XYL eyesight) way. I'm curious...
1. Did you direct-bry the coax? I've had people tell me to go ahead and bury
it. They say that by the time the coax deteriorates I should have replaced
it anyway. On the other hand, you can buy poly hose pretty cheap. That would
keep the lines dry and protect them from an errant shovel or two.
2. How bad is it to run various lines together in a tube. Would I be better
off to run them at least a few inches part to minimize inductive pickup?
3. Of course it would need to be separate but: Has anyone run ladderline
underground? Theoretically it's possible but my intuition says "Don't do
> -----Original Message-----
> From: Donald Chester [mailto:k4kyv at hotmail.com]
> Sent: Thursday, April 20, 2006 12:58 PM
> To: amradio at mailman.qth.net
> Subject: RE: [AMRadio] antenna tuners
> >SWR is not all that bad either as long as the transmitter sees a match,
> >especially the solid state rigs...And no, high swr
> >on a feed line will not cause it to radiate.
> >What causes feed line radiation is an unbalance between the two wires in
> >feed line.
> >High SWR on coax line will cause a little more loss in the feed
> line due to
> >the higher currents involved across the lower impedance of the
> coax. That
> >why when using open wire line that is 400 to 600 ohms, there is much less
> >loss. The same power across a higher impedance means less
> current and less
> >current going through the feed line wire means less power loss.
> Another factor causing loss with a high SWR is dielectric losses at the
> recurring high rf voltage points along the line. At low
> impedances, it is
> the resistive loss in the wire, and at high impedances, it is dielectric
> losses that combine to cause signal loss. But SWR is much, much less
> critical than most hams have been led to believe.
> >An antenna tuner just matches the impedance seen at the
> transmitter end of
> >the feed line to the 50 ohm transmitter output. If there is high
> swr on the
> >feed line, any power reflected from the antenna is not wasted it is just
> >re-reflected back to the antenna and eventually gets radiated.
> >At HF even fairly high swr on coax lines does not cause excessive loss.
> My 160m. vertical uses a 140' run of buried RG-213 from the shack to the
> antenna tuner at the base of the tower. I set the L-network to
> match the
> tower 1:1 SWR at 1900 kc and locked it down. The SWR is about
> 2.5:1 at 1800
> kc and the same at 2000 kc. I have measured the rf power input
> to the tower
> using a General Radio antenna impedance bridge and thermocouple
> rf ammeter,
> and at the same DC input power to the final amplifier, I could not detect
> any significant difference in rf power at the antenna end of the
> across the entire band. Of course at each point I had to re-measure the
> base impedance of the tower, note the rf ammeter readings, and
> per ohm's law, so I do not guarantee precision measurements, but the
> ballpark power was consistent across the band, with no evidence
> of excessive
> loss at the top and bottom edges
> Don k4kyv.
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