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Author Topic: Grounding a 5BTV  (Read 779 times)
KI4SJU
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Posts: 44




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« on: June 25, 2012, 04:53:03 PM »

Ok I know this is a stupid question but I recently installed my 5BTV on the ground. I hammered a 4 foot pipe in the ground and mounted the antenna 4 inches off the ground.  I have a ground rod in the ground beside the antenna and attached the ground wire attached to the U bolts on the mounting bracket. My question is that the proper location to mount the ground or should it mount to the underside of the antenna where you connect the center of the coax to the antenna? Or should I mount the ground to the holes above the U bolt holes? Oh and one more question,  should I mount the radials to the same location as the ground wire or should they be seperated?
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WX7G
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Posts: 6331




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« Reply #1 on: June 25, 2012, 05:19:14 PM »

It isn't critical. The U-bolts make a convenient place to attach radials.
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K0ZN
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Posts: 1563




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« Reply #2 on: June 25, 2012, 08:43:47 PM »


As long as the length of the connection is fairly short, there should be no issues. The worst case is a long distance between where the shield of the coax connects to the antenna and the ground system.

Understand that a single ground rod is virtually worthless as an RF ground for the antenna system.....it is mostly a safety and lightning ground; it is not a good RF ground.
You noted that you were installing radials; that is very good. The radials are literally the other HALF of your antenna system....which is why you need a LOT of them....
they are the REAL ground system for the antenna.

A vertical like yours with only one ground rod for the "ground" will suffer about a 50% loss of the transmitter power in earth resistance losses. i.e. if you put 100 watts into a vertical with only one ground rod, only 50 watts of the energy will be radiated. That is because plain old dirt is a very poor conductor of RF current.... it is very lossy.....and the copper wire in radials is a very GOOD conductor, thus reducing ground losses. A 50 watt signal WILL get out "OK" (it is only -3db from full power), but you are certainly better off with ALL of the transmitter power being radiated.

For decent efficiency, you want a minimum of about 32 radials that are 1/4 wave long on your lowest band. If you have a space problem, more short radials are better
than a few long ones because the RF current is highest closest to the base of the antenna. The radials must run in a true radial line from the antenna; bending them around corners is pointless because the RF current field runs in essentially straight lines.... RF is NOT like DC or low frequency AC, both of which will stay in a wire; RF in this
situation will not. It follows flux lines.

Also note, that if you install only one ground rod and a few radials for the "ground" your SWR likely will be very close to 1:1 with 50 ohm coax feed. As you add radials, the SWR will go UP some...that shows that the antenna system radiation efficiency is increasing. With one ground rod, the ground loss resistance adds the system impedance
bringing it closer to 50 ohms to match the coax impedance of 50 ohms. With a good radial system, the feed point drops to about 36 ohms which yields about a 1.5 SWR. If you add radials until you get about a 1.5 SWR, that would be an indication that your antenna is nearing 100% radiation efficiency.

Most of the complaints about poor performance from verticals is because the ham did not install an adequate radial system. If someone would put a dipole and make one half of the dipole out of plain old dry dirt, that dipole would not work very well either !! Verticals can be very good antennas, but they are labor intensive. i.e. require a lot
of radials.

73,  K0ZN

P.S.   Note to other "experts"; the above is an attempt to address this subject in a SIMPLE manner to help a fellow ham. It is not an engineering design paper and
         no attempt has been made to cover all details/parameters/situations, etc.    -- 73 --


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