[Elecraft] KAT500 Antenna Grounding

[Jim Brown]

On Fri,5/8/2015 5:18 PM, David Ahrendts wrote:
> I’m considering stacking the Cushcraft A503S 6M yagi with my MA5B mini-beam rather closely — like five-feet vertical separation (Rohn five-foot tripod on the roof in a tight lot). Cushcraft recommends using an antenna switch to ground the unused antenna since the two would react with each other at 17M. Both would be connected to the KAT500. So here’s the question: are the un-selected antennas out of the three on the KAT500 grounded  through the KAT500 or do they remain just unattached but active?

Several MAJOR points of confusion here. First, what do you mean by 
"grounding?" Do you mean shorting the coax? Second, because any 
transmission line transforms the impedance by an amount determined 
entirely by its electrical length, shorting the coax in the shack will 
only short the antenna if the line is some multiple of half-waves long 
AT THE FREQUENCY WHERE INTERACTION IS THE CONCERN. Not only that, an 
OPEN in the shack would be transformed to a short at the antenna if the 
line were some odd number of quarter-wavelengths.

So -- if you're going to implement that switch in the shack, you need to 
MEASURE the electrical length of the transmission line to that MA5B to 
pretty good accuracy. If you don't own a good VIA or VNA, find someone 
who has an AIM or VNWA and have them measure it for you. With that 
information, use TLW (comes on the ARRL Antenna Book CD) or SimSmith 
(free Smith Chart program, runs in Java) to figure out how much coax to 
add or subtract to put the short or open.

73, Jim K9YC