[Elecraft] KAT500 Antenna Grounding

Jim Brown jim at audiosystemsgroup.com
Sun May 10 13:07:41 EDT 2015


On Sun,5/10/2015 9:08 AM, WILLIS COOKE wrote:
> Jim, I am afraid that you are ignoring the fact that RF ground and DC 
> ground are not the same thing.

I'm not ignoring that at all -- I'm attempting to help David with an 
antenna interference issue, and understand how transmission lines work.  
AND -- lightning is NOT a DC event, it is an RF event. The energy in 
lightning is broadly centered around 1 MHz. Those feedlines should be 
bonded to the tower at top and bottom, and the tower needs ground rods 
around it.

> Any thing that refers to wavelength of coax is talking about RF 
> grounding.

There is no such thing as RF ground.

> 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.

The appropriate measure of spacing is wavelength, not feet or meters.

>  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.

The wavelength in coax still applies.

>  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 .

The antennas should ALWAYS be grounded.  Again, there is this confusion 
between multiple uses of the word "ground."  The antenna is grounded 
when we bond the coax to earth. That is the first element of protection. 
The second element is bonding all grounds in a premises together (power, 
CATV, telco, etc), bonding all station equipment and antennas together, 
and bonding all of that to the collection of premises grounds . A 
lightning arrestor is the third element -- if the coax is connected to a 
rig, the arrestor shorts the center conductor to the shield in the event 
of a strike, attempting to protect the rig.  Depending on the nature of 
the strike, that may be enough.

>  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.

This is only "good" if the SWR is fairly low. If it's high, the line can 
get pretty lossy.

> Personal protection is usually good enough from the power source  if 
> you have a DC bond to ground

I certainly don't want the Green wire to be the path to ground for a 
lightning strike.  Rather, I want ground rods at the base of the tower 
and where coax enters the shack.

> 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.

Right.

73, Jim K9YC
> 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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