I would like to share some of my experience with small flags'.
The directivity is the same for a large chance in frequency but the gain
increase with size.
As a reference NX4D started with a single flag 14' high 7' wide , using a 20
db gain preamp. Is was good enough to work 150
countries on 160m, for 80m you can reduce the size by 1/2 and expect the same
results. Basically it is a flag like k9AY, EWE, pennant and others
loaded loop, one resistor and one transformer.
The RDF is a limitation and Doug phased two loops 14x7 spaced 16ft for a
total boom of 30FT. all fiberglass, and an isolated mast from the tower and,
not portable, I called small Waller Flag. The two 14x7 flag was good enough
to work over 200 countries from a 1/5 acre lot in a subdivision with a lot of
noise form neighbors. Detuning the TX antenna was a must for good
performance, including working JT1CO direct path over the North pole on
160m.
But the gain of the antenna was a limitation on 160m, and he built a Monster
WF to work 311 DXCC on 160.
2 db NF and 20 db gain is all you need for a vertical flag or dual flag like
the WF(Waller Flag, from Doug Waller, NX4D)
33db gain is too much for a flag or WF vertical, it is good for a flag or a
WF horizontal, and at least 75 ft above ground.
Here is important to remember if you S meter is moving you have too much
gain, I match my preamp gain for s0 on band noise during the day. There is a
lot of signals bellow S0. Keep the gain at minimum.
To reduce common mode noise a twisted pair 100ohms feed like helps a lot.
Measure noise figure at 1.8 MHz is a great task and care, with a good signal
generator and good sound card, you can take one measure in 2 hours of work
for -+ 0.1 db accuracy.
Doug me and Dr Dallas did a long experiment to measure that and a small flag
14'x7', was not thermal noise limited for DX use on low babnds.
At a very quiet rural area with -125 dbm noise floor during the day. For
reference , my city lot average -85 dbm during the day and the best I ever
measured was -95db on those winter Sunday Mornings with light rain.
The Dallas files is not available, Dr Dallas is SK but I have some paper I
can share here.
So a small loop is not thermal noise limited on 160m, and works very well on
80, 60 40 and 30m. some big contest station using a HWF reported problems of
power line noise on 20m and the HWF saved them transmitting on the yagis and
receiving on the HWF. On 80 and 40m the HWF is comparable only to a 4
elements full size yagi. ( same directivity, but vertical noise canceling)
The easy way and accurate is to use FSM easy to do in 3 steps , here >>>
https://www.owenduffy.net/software/fsm/index.htm
FSM (for Field Strength Meter) is a software application that extends a
conventional SSB receiver to allow measurement and calculation of field
strength of radio signals or interference. FSM is a software implementation
of a development of the technique described by Ed Hare of the ARRL in
"Manual Testing of Field-Strength Levels Using Conventional Receivers" dated
August 2004 .
You need a good sound car and a 50 ohm shielded load, like an N or BNC
connector 50 ohm termination.
Regards
JC
N4IS
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