[Lowfer] LF Lop Antennas

[John Andrews]

Tom:

> One thing I am trying to get a handle on is the relative Q of various types of capacitors - Air variable, Poly variable as in what Doug Hendricks has,  mica, mylar, polystyrene etc.   Does anybody have a resource on this?  So far I have seen air variable as in the 1500' range and mica at 1200.  

If high Q is really important to your application, then you will have to 
consider the RF resistance of the bearings. I've done bridge 
measurements on a lot of air variables, and it's not unusual for a dusty 
old cap to show a couple of ohms of series resistance. If you're talking 
1000 pF at 185 kHz, that's a maximum Q of 430, and we usually think of 
caps as being better. I recall using some conductive lubricant years ago 
on some ARC-5 variables, and it made quite a difference.

On that theme, I'd guess that some of the plastic-dielectric caps are 
less Q-limited by their plastic than by their bearings, which are 
usually pretty flimsy.

That being said, there may be an upper limit to the amount of Q you can 
tolerate with a loop antenna, especially if it is some distance from the 
receiver. You'll either spend a lot of time running back and forth, or 
trying to minimize the backlash in a motor drive. Also, given the low 
noise in electronics at LF, and the high noise in the environment, a 
high Q circuit may improve the signal, but not the signal to noise 
ratio, unless the antenna BW is narrower than the rest of the system.

John Andrews, W1TAG