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Daily variance in Pt 15 AM radiation pattern

T

Tom Wells

I have to wonder, as I drive the neighborhood, why my signal strength varies, and pattern, from day-to-day.
Some days it's one or two blocks, other days 3 to 4. The output of the transmitter is not changing,
I expect and hear the other users of the frequency intruding more or less from day to day, but besides that,
my signal seems to vary more than I would expect. I have 1 radial, at 270 degrees, the only direction on narrow city lot I could run it.
And it has pretty big kink in the middle of the radial.
When I check, the output tuning is properly peaked.
I have to wonder if this is just varying ground conductivity.
Is it possibly the upper soil moisture varying and changing the conductivity?
This is in a very dense city neighborhood with brick construction.
 
You can have variables such as ground conductivity as well as "obstacles" which may block or otherwise change the way your signal radiates. Part 15 power is small and I have seen things such as the aluminum siding on a person's home change how well or how poor their station performs. After I mentioned this to the person who had their system near their home and they moved in into the open space of their backyard property their coverage area just about doubled.

A simple test would be to soak your ground system (assuming you have radials) with a hose and check your relative signal strength before and after and note any changes.
 
I did use insulated wire for the radial, and while it is grounded at both ends, I can see there might be an advantage to
having the entire length in contact with earth. There is a proper ground rod at the base.
There is almost no aluminum siding in this 1920's neighborhood. It is also stable, building-wise, no space for new construction.
Houses-or-apartment 2 flats are often 6 feet apart. I can touch my house and the neighbors at the same time.
The antenna is now in this space between houses.
For some reason, this position performed better than "in the clear" of the 30' x 20' backyard.

I've tried to look for a pattern following dry/wet periods, but it seems more random.

I did recently add crystal control to my 1620, and it's wonderful not having to zero-beat the transmitter against
a reference, quite so often. I was very pleased with the stability of my oscillator before. I designed it to go from 1500Khz to 30 Mhz.
I didn't build it with an oven, but did use very a chunky inductor and var capacitor. That cap is 85 years old if it's a day.
It has a VR105 glow regulator, variable L and C, and a tiny trimmer cap for setting critical oscillation feedback,
or cutting back to no oscillation and using a driver reference oscillator, or plugging in a crystal.
Still drifts 10 hz or so, even though the basement floor is always about 72 degrees.
I suppose the heater in the 6V6 could go up and down with voltage, while the B+ is regulated...
I had only ever tested it once years ago with a 3725kc crystal. Who wants to be on 3725?
Anyhow, with the crystal, I am s'prised at how much you can vary the crystal from it's frequency using the "side trimmer" cap.
It really does behave just like a high-Q tank circuit.
I haven't tried to run it way off to see how far, and don't plan to.
 
Hello Tom,

This Ham Radio article (see: http://www.qsl.net/wb3gck/2ndstory.htm ) describes a wire antenna mounted between two close-together (just 8' apart) houses, which also worked well like your Part 15 AM antenna installation.

I experienced similar counter-intuitive Part 15 AM results earlier this evening. I was testing my Talking House transmitter with the remote ATU/whip antenna on the balcony (I found that the flat "under-door" RG-6 coax *just* fits under the outer weather door when closed--yee haw!), playing a DEVO CD for on-air test audio. The ATU meter's needle was on ~8.5 out of 10 on the scale.

On the street outside, the signal faded to background noise just 15 feet directly in front of the plainly visible whip on the second story wooden balcony, yet the signal was loud and clear in front of the opposite (South) side of the apartment building as well as inside (even in the steel elevator!) It was also clear outside the West end of the building and less clear, although still listenable, outside the more distant East end. Perhaps the signal was coupled into the building's wiring?

Also, one of your earlier loop-related postings got me wondering--since the Talking House remote ATU is actually an outboard tank circuit that's attached to the circuit board via the coax, could a large loop antenna (with parallel tuning capacitor) serve as the outboard tank circuit instead? (Some crystal radio designs use a planar [spiral] or edge-wound loop antenna as the L component of the LC tank circuit.)


-- Jason
 
I also have been having issues with signal radiation differences on my part 15.
About a month ago I was achieving a reasonable signal for about 4 blocks or so. Now with the exact same antenna & no changes of any kind I'm lucky to get half that distance. I've tried repeaking & retuning but I cannot
achieve the previous distance which I was getting for a good period of time.
This is all during the daytime as nighttime distances are not reliable anyway.
 
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