I have been creating a back packable yagi for 20 meters and have needed
a means of measuring signal strength into the antenna under test. My
reference signal of choice is the nearby W6WX 20 meter beacon located
about 10 miles to the south. The beacon, part of the international
beacon system, broadcasts carriers at 100 watts, 10 watts, and 1 watt. I
started this effort using the K2, largely for its portability and small
battery requirements. The K2 S Meter with the LED bars seemed like a
poor compromise but was used initially for basic tuning. For fine tuning
I needed a much finer meter. I opted to bring the AGC voltage outboard
of the K2 and employ this as the input to a digital volt meter. This
voltage is driven from U1 pin 1 on the Control board and available
easily on R4 on the same board. What follows are measured results of the
AGC level versus signal input.
Observations:
The voltage levels were quite stable over many evenings of testing with
my reference antenna (a KT34 commercial 4 element yagi). K2 was quite
stable as well as the signal levels received at my location from the beacon.
K2 levels remained quite stable until just before the low battery
indication showed on the display.
Fast AGC was most useful for signals that vary. Slow AGC seemed to
integrate voltage levels too slowly for a signal that is not a solid
carrier.
Here is data that you may find useful for in a similar situation:
Yagi antenna , 3 measurements over 9 minutes, AGC on Fast
Beacon @ 100 w, Front yagi Vagc = 4.23, Beacon at 10 w Vagc = 4.14
delta = 0.09
Beacon @ 100 w, Front yagi Vagc = 4.24 , Beacon at 10 w Vagc = 4.14
delta = 0.10
Beacon @ 100 w, Front yagi Vagc = 4.24 , Beacon at 10 w Vagc = 4.13
delta = 0.11
Beacon @ 100 w, Back yagi Vagc = 4.09, Beacon at 10 w Vagc = 3.99
delta = 0.10
Beacon @ 100 w, Side yagi Vagc = 4.15, Beacon at 10 w Vagc = 4.03
delta = 0.12
24 hours later:
Beacon @ 100 w, Front yagi Vagc = 4.24 , Beacon at 10 w Vagc = 4.14
(several measurements duplicated this result)
In looking at the data one notices that there is nearly always about a
100 mV difference for a 10 db change. Given the MC1350's AGC range this
seems fortuitous. The MC1350 has, according to its data sheet, several
inflection points as agc voltage increases. It appears at least within
the voltage ranges I was operating in, the gain variation was fairly
linear (for decibels) and made antenna front to back estimates quite
realistic. Of course all of this will be different depending on your
adjustments in the circuit or modified values around the AGC circuits.
Conclusion ? You might find this a useful tool for similar measurements,
while certainly this is not a precision instrument, it has allowed at
least one operator to accurately tune a yagi to meet and beat the
performance of a commercial yagi in a similar installation.
brian n6iz.
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