Ground Plane Antenna Question - Grounding of Mast

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KR7CQ

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If you were to take one of the RS 20-176 antennas (or similar) and mount it on a mast made of PVC with a wood core, would this work just as well as the same antenna being mounted on a grounded mast?

Are there cases where any type of antenna (scanner, ham, etc.) needs to be mounted to a grounded metal mast so that the antenna mount is electrically connected to ground?

Any input would be appreciated.
 

WA0CBW

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The NEC (National Electrical Code) requires any metal antenna mast to be grounded to its own ground rod. The antenna ground plane or mount is connected to a ground rod only for electrical/lightning protection and not for receiving or transmitting requirements. Also per NEC code all ground rods need to be connected to the site electrical ground.
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Ed_Seedhouse

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If you were to take one of the RS 20-176 antennas (or similar) and mount it on a mast made of PVC with a wood core, would this work just as well as the same antenna being mounted on a grounded mast?

Are there cases where any type of antenna (scanner, ham, etc.) needs to be mounted to a grounded metal mast so that the antenna mount is electrically connected to ground?

Any input would be appreciated.
So far as radio frequencies the answer is that the radials act as ground and connecting them or the radiating element to a grounded metal mast will ruin performance by changing the impedance of the antenna. Depending on power radiated and the robustness of your radio's finals, that could result in damage to the radio.

But you do need a lightning ground if you live in storm territory. At the very least the outer coax braid should be connected to a good Earth ground. Lightning bypass devices are available as well. I have also heard the suggestion that running part of the coax underground will also act as a lightning bypass, but the coax may deteriorate fairly rapidly if you do that, depending on it's construction.
 

WA0CBW

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Ed,

Could you expand on your comment about connecting the radials to a ground. The ground radials are normally part of the mounting structure and are usually connected to the metal mast. As you indicated the NEC does require that the coax shield be connected to ground before it enters the building. I don't think I saw anything about running the coax underground helping with lightning protection in the Motorola R-56 manual.
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Ed_Seedhouse

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Ed,

Could you expand on your comment about connecting the radials to a ground. The ground radials are normally part of the mounting structure and are usually connected to the metal mast. As you indicated the NEC does require that the coax shield be connected to ground before it enters the building. I don't think I saw anything about running the coax underground helping with lightning protection in the Motorola R-56 manual.
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Since we're in the scanner forum I'm assuming a VHF/UHF antenna. The theory is the same with a receiver as with a transmitter but in practice an impedance mismatch is not a big deal for a modern receiver, so it's not that important. Grounding the antenna directly at the antenna connection can upset the radiation angle as can mounting it on a metal pole unless the pole is below the bottom of the antenna.

In practice grounding the cable somewhere between the receiver and the antenna should be good enough for safety but strictly this does not ground the actual antenna element, only the radials. For that you need to spend some bucks on a proper lightning bypass which will shunt electrostatic fields to ground from both the radials and the quarter wave vertical. You only want a D.C. ground to prevent buildup of electrostatic fields around the antenna. You don't want radio frequencies to go to ground, you want them to go to the receiver. The Radials, if properly designed, are all the RF ground you need.

But it's not that big a deal with a receiver. When we get to transmitters it becomes more important because an impedance mismatch can cause damage to a radio if it gets too bad. Most modern transmitters will prevent the damage by reducing there output before it can happen.

As for running coax underground for this purpose it's something I read a long time ago and probably I shouldn't have mentioned it.
 
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jim202

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Since we're in the scanner forum I'm assuming a VHF/UHF antenna. The theory is the same with a receiver as with a transmitter but in practice an impedance mismatch is not a big deal for a modern receiver, so it's not that important. Grounding the antenna directly at the antenna connection can upset the radiation angle as can mounting it on a metal pole unless the pole is below the bottom of the antenna.

In practice grounding the cable somewhere between the receiver and the antenna should be good enough for safety but strictly this does not ground the actual antenna element, only the radials. For that you need to spend some bucks on a proper lightning bypass which will shunt electrostatic fields to ground from both the radials and the quarter wave vertical. You only want a D.C. ground to prevent buildup of electrostatic fields around the antenna. You don't want radio frequencies to go to ground, you want them to go to the receiver. The Radials, if properly designed, are all the RF ground you need.

But it's not that big a deal with a receiver. When we get to transmitters it becomes more important because an impedance mismatch can cause damage to a radio if it gets too bad. Most modern transmitters will prevent the damage by reducing there output before it can happen.
You sir need to go back to antenna school and also read up on what the NEC is saying on grounding communication antenna systems. Plus your home owners insurance will have a big part to play in this. Best to check with the insurance company to make sure you comply with their requirements.
 

ko6jw_2

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Ground plane vs earth ground

A ground plane antenna works because of an artificial ground. Another name for this is a counterpoise. It is not to be confused with an actual earth ground. Therefore, a ground plane will function just fine on a PVC pipe. Connecting an earth ground to a metal mast is a good idea. It will not affect the antenna, in spite of comments to the contrary. The radials isolate the radiating element from the structure. If a ground affected the antenna, you could not connect coax to the antenna without de-tuning it. If you live in an area with lots of electrical storms, you should put a lightening arrestor in the coax and connect it to an earth ground in addition to grounding the mast.
 

LIScanner101

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Nobody seems to have answered his very FIRST question, which is:

If you were to take one of the RS 20-176 antennas (or similar) and mount it on a mast made of PVC with a wood core, would this work just as well as the same antenna being mounted on a grounded mast?
I’m pretty sure that what he meant by “work just as well” is “will it receive as well”. With that assumption made (and I’m pretty sure I’m right about what this question means) the answer would be it will receive just as well, and perhaps even BETTER, because when the 20-176 is mounted on a metal mast the mast causes the antenna to be slightly directional. However, on a PVC pipe with a wood core, the directionality would be reduced or may even be negligible because there is no longer a metal element near the lower radial.
 

prcguy

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His first question has been answered several times and there should be no difference in performance mounting the antenna on a metal or plastic or wood mast since the ground radials will decouple the vertical element(s) from the antenna surroundings.

If you think about it the coax exiting the antenna will have the same effect as the antenna sitting on a metal mast since its a conductor connected to the ground radials running down away from the antenna.

There are types of antennas where a metal mast will have an effect due to common mode RF currents flowing on the coax or mast. This is usually due to poor design and the RS Sputnik should not have this problem.
prcguy



Nobody seems to have answered his very FIRST question, which is:



I’m pretty sure that what he meant by “work just as well” is “will it receive as well”. With that assumption made (and I’m pretty sure I’m right about what this question means) the answer would be it will receive just as well, and perhaps even BETTER, because when the 20-176 is mounted on a metal mast the mast causes the antenna to be slightly directional. However, on a PVC pipe with a wood core, the directionality would be reduced or may even be negligible because there is no longer a metal element near the lower radial.
 

LIScanner101

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His first question has been answered several times and there should be no difference in performance mounting the antenna on a metal or plastic or wood mast since the ground radials will decouple the vertical element(s) from the antenna surroundings.

If you think about it the coax exiting the antenna will have the same effect as the antenna sitting on a metal mast since its a conductor connected to the ground radials running down away from the antenna.

There are types of antennas where a metal mast will have an effect due to common mode RF currents flowing on the coax or mast. This is usually due to poor design and the RS Sputnik should not have this problem.
prcguy
Whoops!! Sorry :( !! I just realized what I did here! I mixed up the 20-176 (a ground plane antenna) with the Scantenna (a cluster dipole). My error and I apologize for that. Yes, the question has been answered. Again, sorry :(
 

KR7CQ

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I now fully understand the issue with regard to both earth ground, and the "counterpoise ground", and understand the difference. I have always grounded my masts luckily. Thanks for the help guys.
 
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