|[AMRadio] antenna tuners|
John E. Coleman (ARS WA5BXO)
wa5bxo2005 at pctechref.com
Thu Apr 20 19:40:13 EDT 2006
There should be no problem with running coax under ground directly
buried or inside some other conduit including metal and run as many as you
want side-by-side. Balanced line is another thing. Any transmission line
will give you trouble if the characteristics of the insulation is not
constant throughout the length of the line. In the case of balanced line
the electromagnetic fields will be most intense between the lines but will
also surround the lines so that any discontinuity in the surrounding
material will most defiantly cause the characteristics of the line to change
and some materials are very susceptible to absorption of RF energy even
though they are insulators to DC. Obviously conductive metals near the line
or a real no-no. I would guess that open wire balanced lines even if made
with insulation around it and buried underground would cause ground heating
and a lot of RF loss as opposed to hanging nicely from a doublet to a tuner
is nearly with out loss.
I once had a 75 meter doublet (120 ft) center fed with coax and
changed it to the 60 ft of open wire down line. The antenna center was
supported by a 50 ft pushup pole on the top of the house. The 60 feet of
open wire down line would just reach to the edge of the roof and under the
roof edge where the balanced tuner was mounted. It tuned perfectly but
after about 3 minutes it would need to be retuned after about the 3rd time I
found that the #12 wires of the open line were melting into the composition
roof at the edge of the roof where the wires touched the roofing material.
By the way, the wires had the regular plastic insulation on them that
regular electric wires have. They were dielectric heating the composition
roof material and melting down into it. I fastened a piece of Plexiglas to
the roof so it would stick out from under the roofing material and cut
notches in it for the wires to drop into and this solved the problem.
I did an experiment once with a dummy load placed at the other end
of the back yard. The dummy load had a self contained RF voltmeter on it.
I ran 100 ft of brand new RG 8 to it from the connection of my SWR Bridge
and loaded the rig to 2500 Volts at 400 ma on 3885 KHZ. The RF voltmeter
said 185 VRMS at the 50 ohm load. The SWR Bridge sensitivity was up all the
way and showed no reflected VSWR. I then ran 60 ft of open wire line from a
Johnson KW match BOX to the 50 ohm (in place of the 100 ft of coax) and
connected a 10 FT coax line from the VSWR bridge to the Match BOX. I
adjusted the MATCH BOX until the VSWR reflected was zero with the
sensitively up all the way. The rig tuned exactly the same as before and
all meters measured the same except the RF voltmeter measured 195 Volts RMS
across the 50 Ohm dummy load. So I would say that proves that 60 ft of open
wire line and a Johnson KW MATCHBOX has less loss than 100 Ft of RG 8 at
During the same experimenting I found that the MATCHBOX did not like
to be connected to the 50 ohm load directly using the balanced line output
terminals on 3.885 MHZ. The impedance was to low for 75 meters. When
connected to 45 to 75 ft of open wire line and the other end to the dummy
load it would tune nicely on the 75 meter band and was very efficient. It
did not like going over 80 ft on 75 meters of line and started acting as it
did before when it was connected to the dummy load directly. It would
probably start tuning right again if I had increased the line length to
about 180 ft +/- 20 ft but I ran out of backyard as you need to suspend the
line in a fairly straight line to make these tests or I would have had to
stick poles in ground to hold up the line as I made turns around the yard.
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of W1EOF
Sent: Thursday, April 20, 2006 2:51 PM
To: Discussion of AM Radio
Subject: RE: [AMRadio] antenna tuners
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
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