On Mon, 15 May 2006 08:26:39 -0400, Sonny Hood wrote:

ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna
for 75 meters. The antenna wire is #12 multi-stranded tinned copper
with 600 volt TEW insulation. The antenna is fed within one foot of
the NW corner, via 25 ft. of 450 ohm twin lead to a 1:1 balun then via
air dielectric RG/8 (21.1 ft.) then to a AT-1500CV. The formula 1005/f
was used to figure the length and to cut the wire. However, due to
the low height about ground (24-26 ft.); the ground capacitance
changed the antenna resonance considerably. The target f (frequency)
was 3.9 MHz with 257.7 ft. of wire. After installation in a rectangle
configuration at 25+/- feet, the resonate frequency was 3.452 MHz.

You would expect the feedpoint impedance of a full wave loop (cut
using your formula) to be around 90 ohms at resonance.

You have then connected 25' of 450 ohm line. Lets assume this is
Wireman 551 line, the input impedance to this line section at
(assuming 3.7MHz) should be around 141+j282 (and line loss would be
~0.1dB).

You then have an unspecified length of "air dielectric RG/8" (new to
me) which will transform the impedance further.

You seem to intend to then use an ATU to transform this impedance to a
satisfactory load for the transmitter.

Where did you make the measurements with the MFJ269? If it was not at
the antenna feedpoint, did you really expect resonance at the
measurement point to coincide with loop resonance, and is resonance at
either point important to efficient operation of the entire system?

Owen

With a tuner matching the antenna to the "rf" generator (transceiver),
the NVIS signal met the 100-200 mile range that was estimated.
The antenna was tuned - cut (shortened) to increase the
resonate frequency into the 75 meter phone band. After some
estimating of desired length at the low elevation and cutting and
pruning a resonate frequency of 3.790 MHz was obtained, that has a
VSWR of 1.6 to 1. The resonance was found with an MFJ-269 without the
tuner, 1.6:1 @ 3790 KHz. The tuner matches the load to 50 ohms for the
PW-1. The area of the loop decreased and the perimeter length is 229.5
linear feet. Again with the use of a tuner throughout the phone band
produced a NVIS signal range that was very good. K4WYS

On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and
screwed up the resonance, does anone know of such an animal?
K4WYS
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