[Elecraft] KAT500 Antenna Grounding

[WILLIS COOKE via Elecraft]

Jim, I am afraid that you are ignoring the fact that RF ground and DC ground are not the same thing.  Any thing that refers to wavelength of coax is talking about RF grounding.  Anything that refers to lightning protection or personal protection refers to DC grounding.  Anything that refers to antenna spacing refers primarily to interaction between the antennas.  For interaction between the antennas a five foot mast is possibly enough.  If you have an option for more, go ahead with more spacing.  The widest spacing that I see is about 10 feet, but try what you think and see what you find.  You can always extend the mast if you have to because a six meter beam is small and the extension can hold a lot on a 1 inch mast that is 5 or 10 feet long.  It is easy enough to add a coax switch such as an MFJ-1702 Which has an alternate selection for a dummy load or second transceiver which you can install on each of your antennas.  If the antenna is connected to your amplifier or transceiver there is  surge protector which will protect your equipment to a certain extent if there is a strike when you are operating and a ground position when you are not operating or using a different antenna.  You can also use the center position to ground your antenna to the station ground to check for interaction with the other antenna.  If the SWR improves or your signal to the other station improves enough to consider, then you have a strong clue that you should ground that antenna when you are using the other antenna .  Your path to the station ground must be low enough for the strike you are considering to bleed off without harming your equipment.  The coax wave length property is a good one to use if you are having trouble withe SWR or Tuner settings.  Personal protection is usually good enough from the power source  if you have a DC bond to ground and watch what you touch because a direct strike will cause many amps of current in the ground path and only a few milliamps are needed to harm you or your equipment. Willis 'Cookie' Cooke,TDXS Contest Chairman K5EWJ & Trustee N5BPS
      From: Jim Brown <jim at audiosystemsgroup.com>
 To: elecraft at mailman.qth.net 
 Sent: Saturday, May 9, 2015 1:48 PM
 Subject: Re: [Elecraft] KAT500 Antenna Grounding
   
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
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