[Lowfer] Beacon "IO" QSY to 185299.2

[Garry Hess]

Since no one else has commented on it I must point out that beacon "SIW" 
has been running at 185299.4 Hz since Feb 5 (see the announcement on 
this mailing list). To decide on a frequency the IO operator should look 
at the great capture Jay submitted this morning showing all beacons 
currently QRV near the watering hole. XR and CV do fine at only 0.4 Hz 
separation but I think 0.2 Hz is too small, unless perhaps the 
oscillators are GPS locked and QRSS60 speed is used.

On a separate matter raised by JB (VE3EAR), clearly his tapped 
variometer does very well. That's the way the article "Basic 1750m 
Transmitting Antenna" by Mitchell Lee shows the TX to Antenna connection 
in the first cartoon figure labeled "Antenna Coil". However, in Figures 
2 and 3 of the same article, the equivalent circuit seen by the TX is 
viewed as a series connection of a resistance (the sum of coil 
resistance, ground resistance, and radiation resistance) and capacitance 
  (which is to be resonated with the coil). Since the total loop 
resistance with reasonable ground conditions is on the order of 50 ohms 
there appears to be no necessity to autotransform the impedance by 
tapping into the coil or coupling to it with a secondary loop. Thus, the 
current SIW connection between the TX and antenna is through a coil with 
no tie to ground; it is simply a series inductance that resonates the 
antenna capacitance. The input side of the variometer is tied via a 1 uf 
capacitor to the common collectors of the class-D 1 W amplifier for 
DC-blocking purposes (see, for example, the schematic on W1TAG's site) 
and the output side of the variometer is tied via an RF ammeter (as 
described by W5JGV) to the antenna connection. A large resistance is 
placed across the antenna connection to ground to bleed off static 
charge that might otherwise build up.

Interestingly, it was a challenge to figure out just what capacitance 
the antenna offered. The basic structure with tophat was presumed to 
offer something on the order of 300 pf. However, attempts to measure it 
with an LC meter failed. Thinking the meter methodology might not work 
well at LF, a Wheatstone bridge was built. That should have allowed both 
C and R of the equivalent circuit to be measured, but it didn't work 
either. After a lot of head scratching the antenna connection was 
examined with an oscilloscope. Although the antenna is at least 100' 
from any power line, it nonetheless was producing a rough 60Hz waveform 
of 100 V peak-to-peak! So much for why low-level techniques didn't work. 
What did work was a high-power signal generator and SWR bridge. They 
determined the equivalent antenna capacitance was 376 pf and the 
variometer was a bit too large to resonate that at 185 kHz. Skipping 
over a few turns was all that was needed.

73, Garry, K3SIW, EN52ta, Elgin, IL