[AMRadio] Antennas for AM

Tue Nov 15 11:39:15 EST 2011

Hi Don:

Thanks for taking the time to articulate your findings.

Kindest regards Jim K9AXN

-----Original Message----- 
From: D. Chester
Sent: Tuesday, November 15, 2011 10:23 AM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Antennas for AM

Ever since I was first licensed, I have always fed my wire antennas with
open wire line.  Up to now, the OWL has always been operated as tuned
feeders.  I have built many balanced link-coupled tuners using the classic
circuit with a split stator resonating capacitor, using parallel or series
feed as appropriate. Designing and constructing an efficient tuner to feed a
FLAT /UNTUNED balanced OWL is a completely new experience for me, after
nearly 52 years on the air. I find this a  little different from the
familiar tuned feedline matching network, with somewhat of a new learning
curve.

I just completed two prototype matching networks, one to couple the balanced
4-turn link output coils in each of my homebrew transmitters to the untuned
438? open wire transmission running to the dog-house at the base of the
tower. I experimentally tried several configurations including a
link-coupled tuner with various L-C ratios, but what worked best, with the
best efficiency and least critical tuning, turned out to be a simple
balanced L-network. I could have easily calculated the balanced network as
two separate L-networks working in tandem, one on each side of the balanced
link-to-balanced feeders, with the coils positioned to minimise mutual
coupling between them.  But instead, I used a single split coil, with the
transmitter's link directly feeding the gap at the mid-point of the split
coil, and a split stator capacitor in parallel with the entire coil, with
the feed line in parallel with the split stator cap and coil.  I came up
with the best results using trial and error by moving taps around on the
coil; I did not attempt to calculate in advance because I was not sure if
the normal L-network formulae would work when there is mutual coupling
between the two coils.

With the best configuration, I could load the HF-300 rig up to full power,
modulate up to 130% positive, and the dual 900/900 pf variable cap I used in
the balanced L-network (0.07", 3 KV spacing) never arced over. At about 750
watts carrier output, the rf ammeter read 1.3 amps into the 450? dummy load
with the transmitter link about 80% meshed with the main coil. The overall
DC input to RF efficiency of the final calculated to 75% directly into the
dummy load, and about 74% when I moved the dummy load out to the far end of
the OWL. The mismatch between the 450? dummy load and the 438? OWL is so
slight that it should cause negligible loss or tuning difficulties.

This appeared to be substantially more efficient on 160m than my present
configuration, running the link, resonated with a network of parallel
capacitors in a capacitive voltage divider configuration, to feed a 50?
dummy load or the present unbalanced feed line that runs to the dog-house.
In the latter case, the transmitter loads up fully with the link only about
40% meshed on 160m and the link runs hot to the touch after a few minutes of
steady carrier. Apparently, with so few turns, resonating the link with
parallel capacitance to get the transmitter to take a full load at that low
a frequency, causes the Q and circulating current to be so high as to
produce substantial loss in the link coil.  Overall efficiency with the old
configuration calculates to about 60% on 160, and slightly higher with a
simple variable capacitor in series with the same 4-turn link on 80 and 40.

I won't attempt to use the balanced L-network with the Gates broadcast
transmitter, since it has an unbalanced pi-L output, and the input to the
tuner would necessarily be grounded at one side without the use of a balun,
which I prefer to avoid.  Instead, I threw together another prototype tuner,
using off-the-shelf junkbox parts on hand. This one is a link-coupled
balanced tuner with split stator capacitor, symmetrically tapping down on
the coil to feed the 450? resistive load. I experimented with number of
turns on the link, number of turns across which the 450? line is tapped, and
total number of turns on the coil.  I didn't bother to calculate the
inductance of the coil, which is about 4" in diameter with around 80 turns,
an edgewound coil taken from the variable inductor in a parted-out RCA
BTA1-R broadcast transmitter. The coil originally had a massive moveable
ferrous (or was it copper?) slug in the interior for varying the inductance,
which I long ago removed.  To get a perfect 1:1 match out of the Gates
without resorting to a variable capacitor in series with the link, I ended
up with 5 turns on the link, and about 13 turns (IIRC) between taps on the
main coil to the OWL, using the ENTIRE COIL in the main parallel tuned
circuit. It resonates at 1900 kc/s with only about 35 pf (70 pf per section)
across the coil, and gives excellent efficiency for a class-C tube final
(76% total DC input to rf at the dummy load). But the thing is practically a
Tesla coil! A 1/8" spacing(4500v per section) capacitor arced over
immediately.  I substituted a 3/8" spacing (11kv per section) 100/100 pf
split stator capacitor, and it still occasionally arced over on voice peaks
at 130% positive modulation with the carrier running 750 watts. I have a
1/2" spacing cap (13kv/section) on hand that would probably work, but I may
eventually try doing the maths to see if I can come up with a better L-C
ratio with a more reasonable coil while still avoiding series capacitor at
the link, although any change from the above settings of the tap and turns
in the link in the trial-and-error version either produced lower efficiency
or would not tune to a 1:1 match to the Gates.  However, since I can't
complain about the efficiency of the beast, and the tuning is not overly
sharp or critical, and it maintains a good match over the entire 160m band,
I might just use the thing as-is in all its glory with the big coil and
widely-spaced tuning capacitor.

Don k4kyv
_______________________________________________________________

This message was typed using the DVORAK keyboard layout.

http://www.mwbrooks.com/dvorak/
http://gigliwood.com/abcd/

______________________________________________________________
Our Main Website: http://www.amfone.net
AMRadio mailing list
Searchable Archives: http://www.mail-archive.com/[email protected]/
List Rules (must read!): http://w5ami.net/amradiofaq.html
List Home: http://mailman.qth.net/mailman/listinfo/amradio
Post: AMRadio at mailman.qth.net
To unsubscribe, send an email to amradio-request at mailman.qth.net with
the word unsubscribe in the message body.

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html 