[Lowfer] Lowfer TAG

[John Andrews]

I fired up Lowfer TAG from Raymond, ME on 185.302 kHz, sending TAG in 
QRSS60 this weekend. I have rebuilt the tuning unit at the feedpoint to 
the loop antenna. The plastic enclosure on the old one was being held 
together by duct tape. I also found that some of the capacitors used to 
tune the loop had developed a little series resistance, as measured on 
an RF bridge. The end result was slightly higher antenna current, but 
not enough to make much difference.

I will probably let it run for about a week, unless I advise otherwise 
here. Reports, though possibly unlikely, would be appreciated.

Regarding the capacitors, a little old-time AM broadcast lore: I used to 
work for a 5 kW AM station (WTAG), and the 4-tower directional array was 
fed through tuning networks dating back to 1939. The capacitors were old 
micas, ranging from G1 to G4 in size. From time to time, a cap would 
overheat and die, sometimes explosively. To avert such a calamity, we 
began checking for mild heating, and testing suspected problem caps with 
our General Radio 516 RF impedance bridge. The rule of thumb was that 
any cap showing more than 0.1 Ohm of RF resistance was replaced. That 
greatly reduced the incidence of failures.

When I originally built TAG's first tuning network, I checked the mica 
capacitors, and used ones that met that criterion. But after many years 
of operation, I found this month that a .005 uF showed 0.3 Ohms, and a 
.001 uF showed 0.2 Ohms. When you are dealing with a loop with only 1.4 
Ohms of RF resistance, that's not a good thing. So I replaced the mica 
caps, and put in a spare 1000 pF vacuum-variable in place of an old 
3-ganger. The resistance of the net 7000 pF tuning is now definitely 
under 0.1 Ohms.

As W1VD can testify, this is not your average RF impedance bridge, and 
is capable of fine work. My experience with VNA's has been a little 
mixed with high reactances at LF, so I'm sticking with the bridge.

Of course, most LF work is done with loaded verticals, not loops, so 
stuff like this is not of much interest. However, here in the Maine 
woods with 100' tall pine trees all around, the losses with verticals 
are huge, hence the loopiness I picked up from the late Bill Ashlock.

John, W1TAG