[Lowfer] Lowfer TAG

[John Andrews]

JR,

While the TAG transmitter has been powered off for half of each year, the loop stays connected through the thunderstorm season. I’m betting that the antenna has been “tickled” more than once, with the caps in the circuit. That’s probably been a bigger possible cause of degradation than the 0.9A in the loop when the thing is running. 

Bill Ashlock used to amaze me with his use of dumb old 500v silver micas with his loops. Not rated for a lot of current, I fear. 

JA

> On Sep 24, 2023, at 9:46 PM, jrusgrove at comcast.net wrote:
> 
> John
> 
> Glad to hear TAG is back on the air! 
> I'll take a look for it in a few days ... e probe is currently TU likely due to summer thunderstorms and needs repair. 
> 
> Yes, the GR 516 is quite the instrument even by today's 
> standards! Found your story about micas developing 'high' resistance 
> interesting. Probably a good idea for all stations using loop antennas to check 
> them on a regular basis.
> 
> Jay W1VD
> 
> 
> 
> 
> 
> ----- Original Message -----
> From: John Andrews <w1tag at charter.net>
> Reply-To: Discussion of the Lowfer \(US, European, & UK\) and MedFer bands 
> <lowfer at mailman.qth.net>
> To: Discussion of the Lowfer (US, European, &amp, UK) and MedFer bands 
> <lowfer at mailman.qth.net>
> Sent: 9/24/2023 7:10:31 PM
> Subject: [Lowfer] Lowfer TAG
> 
> 
> 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
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