WA2XTF beacon, Vienna

[Bob Bruhns]

Hal and I got the beacon going at low power Monday afternoon (2002-05-27).  The directional wattmeter was helpful.  I don't know whether Hal left the beacon on, it may still be on.

I discovered that I have been setting the beacon to low power all this time.  High power mode misbehaved so badly that I thought it was low power mode.  Yet we got good signal reports last time we had the beacon on the air, and we also managed to burn up the low pass filter at low power, even though the transmitter appears to have been operating cleanly.

On the transmitter, the green light is ON at high power.  My guess was wrong; I had not read the manual, but it turns out it is no help.  The only way to be sure was to test into a dummy load, which I finally did today.

At Sandy's suggestion, we had changed the configuration of the antenna base tuner away from transformer coupling at the antenna base to an adjustable series loading coil at the antenna base some months ago.  The loading coil tunes the antenna to resonance, then RG-8 50 ohm cable (with some mismatch) connects the antenna to the house, and a matching autotransformer inside the house matches the feedline to 50 ohms for the directional wattmeter, low-pass filter (if used) and the transmitter.  This is how the Vienna beacon is operating now.

I could not get the transmitter to work with the antenna in high-power mode.  I believe this is because the present matching system does not produce a precise match.  However, I was able to get it close.  I think a few additional taps on the matching autotransformer will allow a good enough match for high power operation, and I will try to add those taps this week.  (Easy enough, but who has the time?)  Optimum match is a step-up connection with the transmitter output voltage multiplied by about 1.4 and fed to the antenna feedline.  This suggests the impedance at this feedpoint is about 100 ohms. 

This afternoon the input filter core in the low-pass filter got hot at low power, with evidently clean transmitter output and fair antenna tuning.  But I determined and demonstrated that in low power mode, with fair antenna tuning, the transmitter operates just as well without the filter as with it, and a little more power gets to the antenna without the filter.  In high power mode, the transmitter does not work with or without the filter.  (It seems as though there is a high-pitched amplitude oscillation; I assume it is the protection circuitry oscillating.)  So I set the beacon up without the filter for the time being.  When used, the filter was in the 50 ohm section, before the matching coil.  We had TX, low-pass filter (if used), directional power meter, matching autotransformer, feedline, loading coil, antenna.

I can get the transmitter to work in high power mode on the antenna if I severely misadjust the matching autotransformer.  It does not run full power that way, and I would not want to run the beacon that way anyhow.

At low power, the antenna loading coil must be set off-peak.  I found I could not simply adjust the antenna loading coil for peak antenna current, I had to set it counterclockwise about 30% less than peak, for minimum reflected power indication.  (I don't know if this is more or less inductance.)  At the peak, in low power mode with no filter, antenna current was 1.4A at low power, but at the point of minimum reflected power, antenna current was 1.0A.  Transmitter current was approximately 4.2A DC.  This setting was the best I tried; and with the best matching autotransformer tap at hand, I saw full-scale on the FWD meter and 12% reading on the reflected meter with or without the filter.

On Monday evening (2002-05-27) it rained, and the reflected power indication went up from 12% to 25% scale (with forward at calibration level, 100% scale).  I tried adjusting the matching autotransformer, but it did not help.  I did not mess with the loading coil, because there was lightning as well as rain...  The transmitter was satisfied with this in low power mode, but I want to do something to keep the water farther away from the loading coil.

The transmitter likes the dummy load in low and high power modes.  I think if I can get the antenna tuned and matched to 50 ohms, the beacon should be able to run high power.  The low pass filter may be necessary, as seen previously; if so, the ferrite-core heating problem will have to be solved.  (Oil immersion?)  But if rain detunes the loading coil, the match will be lost.  Hal has a storage shed, he suggests we assemble that around the loading coil.  That should do it.  Maybe next weekend.

Fire hazards...  We have to make sure nothing is getting hot in operation.  The plastic loading coil form may be a problem at high power.  Also plastic immediately around the coil may be a hazard.  We will see.

  Bob, WA3WDR