Idea time.
Idea time.
My present SStran transmitter at 1680 isn’t grounded, but my 3-meter antenna arrangement on the porch is effective and the signal is good for several blocks. I worry about this at times, and when I see white panel trucks I often switch over to the other SStran at 1550, which has a dimmer antenna. That is, a less productive antenna.
If I were to add a (legal) ground plane to the stronger unit, who knows how good it would be. During the bright of day the signal might go a mile. But of course at night there is all the sky interference and no problem.
So at last I come to the point. What if a good ground plane were connected to the transmitter with a potentiometer which could be dialed weaker for daytime but stronger for nightime?
THAT would be like a nightime power boost but it would still be (we hope) within allowed limits.
Ungrounded TX
Well in effect it is grounded simply because of the common ground within the transmitter itself, thus your power and audio ground leads are comprising your counterpoise.
Here is a test to validate that. Isolate the transmitter completely by disconnecting the audio and power wiring. Take a batter pack and plug that into the transmitter. Then test the signal range. I am sure you will notice a major difference.
Or you can simply take a DVM in continuity check and go from the ground of the audio or power to the ground of the antenna connector. BEEEEEEEEP. 🙂
RFB
True
Yes, RF burns, the SStran actually has some jumpers for either grounding through the audio or power supply lines, or isolating those through inductors, and the difference is very noticeable.
But that being true, I admit I am taking it for granted that a set of radials buried in the earth would be a large improvement and extend the signal greatly.
If I am right, could a variable resistor be placed in line with the radials to reduce their influence during peak daytime signal, to avoid too large a signal?
Adjustable Ground
If I am right, could a variable resistor be placed in line with the radials to reduce their influence during peak daytime signal, to avoid too large a signal?
Now this would be an interesting experiment! Naturally by having a variable resistance in the RF ground return path, that will no doubt change the resonance and performance, thus change the signal strength and coverage.
I think the better method to this approach would be to have a dual B+ voltage rail ran to the final, selectable through a mini relay. Perhaps reduce that set B+ currently in the unit by 1/2 or 1/3 with the use of a voltage regulator on the lower B+ setting. This way you maintain your antenna resonance without altering its ground system. However even changing the B+ to a slightly lower voltage results in a change in output impedance of the final circuitry in the transmitter because it is designed to be of certain impedance at a design B+ voltage. Some experimentation will be required here. It may require a change in bias to the final, the loading circuitry on the final etc. It may get too complex for the transmitter being used, plus obvious major modifications to it that will go beyond the point of reason and practicality.
But for a quick solution to day/night settings as your trying to accomplish, I say give it a try with the adjustable ground scheme and see what happens.
RFB
First thing I thought of …
“Naturally by having a variable resistance in the RF ground return path, that will no doubt change the resonance and performance, thus change the signal strength and coverage.”
That’s what came to mind. Peak resonance will undoubtedly be affected. I’d think the system would have to be retuned from one situation to the other.
Yes/no?
Re tweak and peak
Peak resonance will undoubtedly be affected. I’d think the system would have to be re-tuned from one situation to the other.
Yes. Changing the ground, be it through a variable resistor (preferably use a wire-wound variable and not the standard type), or a change in final B+ voltage and bias will change the final impedance and entire antenna resonance and will require re-tuning. This will be even more crucial if you have your antenna system highly peaked and tweaked with little bandwidth range. (narrow)
From Carl’s post:
I remembered how the big stations do it: they switch transmitters (and antennas?) Well, one station I know has its 5kW daytime setup in one location and their 1kw nighttime transmitter and tower in an entirely different part of town, so that their nighttime signal is closer to the population.
Most do have a daytime TX at high power, and a 2nd TX unit for nighttime at a lower power, both usually feeding an antenna combiner/splitter that would change over to the night pattern as the station switches from the high power TX to the lower power TX.
Back in the day it was mostly just 1 transmitter that had the capability of daytime and a nighttime power range. And if the station was required to change the pattern, the single transmitter would feed the antenna combiner/splitter and change the pattern in between the two different power ranges. Often taking about 5 seconds or so.
For those stations that do not require a pattern change, they simply hit the “low” button on the panel or remote control at the time of power change and back again for daytime. Normally taking 2 seconds or so depending on the transmitter.
In either of these cases, there was always a change in tuning which would be accomplished within the antenna tuner unit in the “dog house”. Extra control wiring would run from the transmitter hut out to the dog house for that purpose and work a set of either vacuum contact relays or RF “blade” type contacts via solenoids. During this time there is no RF power in the system and there would be interlock contacts in the antenna tuner so that no RF would be applied unless the proper sequence of these interlock contacts were completed, avoiding any mistake of RF being applied to the system in the middle of changing to the proper tuning for the given power level.
In any case Carl, the antenna system will have to be re-peaked when changing the ground condition via the variable resistor. However do not let that get in the way of trying out the idea. There may not be that much of a change to have to re-tune and the offset of resonance may also help achieve the goal your after.
And if it does, you can go one step further and add a variable cap on your antenna controlled by a servo so that it re-tunes accordingly when its time to change the ground variable and maintain that peak resonance for both scenarios of night and day configuration.
RFB
Will Do
RFburns yes, thank you, I will go ahead and try the idea to see what it does.
At the same time, while I was reading all your cautions about how difficult it really is to change power with a single transmitter, I remembered how the big stations do it: they switch transmitters (and antennas?) Well, one station I know has its 5kW daytime setup in one location and their 1kw nighttime transmitter and tower in an entirely different part of town, so that their nighttime signal is closer to the population.
I hadn’t thought of the re-tuning part of the problem.