Topband: Re: Beverage: Sloping vs. vertical drop

[Tom Rauch]

One comment on sealing boxes. It is virtually impossible to seal a 
box or cable against air leakage. That is why even special gasketed  
transmission lines with solid copper outer conductors are kept under 
constant pressure. 

All sealing a box does is guarantee problems over time with moisture 
ingress.

I hang many or most of my transformers and termination resistors 
right out in open air, taking care only to shield the connector rear 
from rain. I coat the rest with a good plastic spray, to prevent 
corrosion. I have no trouble at all with moisture or connections.

My electronics are not sealed at all, just "hooded" to prevent rain 
from hitting the stuff inside. As a matter of fact, a tight seal 
would virtually guarantee problems over time. 

> Do you mean by this that the ends should not use common 
> ground points for feed/termination, or that the ends should not 
> be in proximity to each other?

It is a good idea not to share ground connections, unless you have a 
very good ground at the termination. It is also a good idea to not 
bring drops down close to each other, a couple Beverage heights 
distance apart should be OK.
 
> I have two Shorty 2-wire Beverages that are fed ~1.5 feet from each
> other.

The less directional the antenna the less noticeable any unwanted 
coupling would be. Think of the problem as the depth of the null of 
one antenna compared to the response of another antenna in that null 
direction. If you had a really deep null, say 25dB, even a very small 
amount of capacitance between two 600 ohm systems could cause a 
problem.

In 50-ohm relay systems, I typically measure about 35dB of isolation 
between an open port and a used port in a good layout using DIP 
relays. Naturally this problem gets proportionally worse as you 
switch higher impedance circuits. One way to test the relay box would 
be to open the selected antenna at the feed end, and replace it with 
a resistor and see how many signals you can hear. Myself, I'd just 
avoid switching high impedance lines.

73, Tom W8JI
W8JI@contesting.com