With an SSTran trasnsmitter and the designs for the base loaded
antenna do you guys think its a good idea to use a variable
capacitor to fine tune instead of the adjustable vertical
radiator?
I am just planning, but I was about to order this one gang
variable capacitor, 14-365PF at http://www.oselectronics.com/ose_p96.htm.
I got the idea for the variable capacitor in Kyle Drake's
LPAM Handbook, but I dont hear alot of people using them.
With an SSTran trasnsmitter and the designs for the base loaded
antenna do you guys think its a good idea to use a variable
capacitor to fine tune instead of the adjustable vertical
radiator?
I am just planning, but I was about to order this one gang
variable capacitor, 14-365PF at http://www.oselectronics.com/ose_p96.htm.
I got the idea for the variable capacitor in Kyle Drake's
LPAM Handbook, but I dont hear alot of people using them.
I'd rather use this than the adjustable radiator, if it will work.
Thanks
Mark
Hi Mark,
This should work OK but you will have to wind the loading coil with extra turns to produce a higher than usual inductance. In practice, I would suggest the variable cap. be placed in series between the coil and the transmitter. Theoretically, it makes no difference, but the junction of the coil/antenna is a high voltage point. Wherever you put the cap, make sure it is insulated and the adjustment can be done through an insulated shaft or other means where you will not be part of the circuit. Adjustiment will be a back an forth deal. The var. cap. is to resonate the antenna and coil, the the SSTRAN cap is to set the correct impedance transformation.
Water has a very high dielectric constant so if this is for an outdoor unit take care to keep water and condensation out of the cap. A little water goes a long way to disrupt the capacitance from the desired value.
I use a var. cap. in my antenna, but it is not the series resonant configuration that is commonly used. The tuning is quite sharp, and if you can, use a field strength meter to set the peak.
Neil
If you could post a schematic or describe the hook-up, that would be kewl!
Experimental broadcasting for a better tomorrow!
Here is a description of my antenna from a previous thread:
http://part15.us/node/494#comment-1226
In that post I mentioned a range of a few hundred feet, but since then I have changed to a quiet X band frequency. With the SSTRAN transmitting from this antenna and using my truck receiver I can hear my signal clearly .2 miles away, and a very fringe signal about 1.5 miles away. With a low $ GE portable receiver, the range is only a few hundred feet but it suffices for my yardcasting. Formerly my truck radio test range was limited by QRM from other stations, but this new frequency is totally quiet during the day.
I do not claim this is a good antenna design. I actually built it for another purpose but it turns out that it works for me on part 15 AM.
Neil
Thanks for finding the other post. I had forgotten that.
It appears that I am contradicting myself about the var. cap. placement but I am not. The 717 post is referring to the tuning cap connected between the bottom of the antenna coil and ground which I see as a potential problem, whereas my first answer in this thread refers to the var. cap. in series with the coil.
Hope this helps.
Neil