Hi.

When grounding an antenna for sending/receiving in the 69 - 78MHz range, how long must the rod be?

 

When I worked in the communications field we used the plated 10 ft grounding rods, 5/8" dia.
go as deep as you can with a fence post driver or a big sledge.
Is this antenna on a pole or a tower?

 

When I worked in the communications field we used the plated 10 ft grounding rods, 5/8" dia.
go as deep as you can with a fence post driver or a big sledge.
Is this antenna on a pole or a tower?

Sometimes, it will be on a pole, and other time it will be on a vehicle. When on a vehicle, the vehicle needs to be grounded.

 

the vehicle needs to be grounded.

No, the vehicle is the ground.

so, I'm guessing this is a mobile antenna that is also used at home.

is it magnetic base? what model/brand is it?

 

No, the vehicle is the ground.

so, I'm guessing this is a mobile antenna that is also used at home.

is it magnetic base? what model/brand is it?

How come the vehicle is ground, with rubber tires? I've read somewhere that to reduce noise and enhance the signal, it is necessary to ground the antenna. And I was wondering how long the grounding rod had to be.

 

You are, I gather, trying to provide an RF ground for the antenna, not an electrical safety ground. At 70 MHz, a 1/4 wavelength radial would be approximately 1.02 meters long. Four or more of these radials should improve the antenna's performance. They are arranged perpendicular to the vertical portion of the antenna. I refer you to any good reference on antennas, e.g. the ARRL Antenna Handbook, any edition. (Likely available at your local library.) For this purpose, a ground rod driven into the earth is not required, but probably won't hurt. If you are using coaxial cable as a lead from the transceiver to the antenna, the center conductor goes to the vertical element, the shield to the radials and/or ground rod.

In a mobile installation, the frame and other metal parts of the vehicle provide an RF ground, coupled by capacity to earth ground. Not the best of all possible worlds, but it's what is available.

 

Yes, an RF ground... Should have said that in the first post...

So it won't hurt to put a rod in the ground. 10cm or 60cm. What is the exact function of that rod?

 

K7GUH is right on!

Unfortunately the term 'grounding' has more to it than meets the eye and in fact its name 'grounding', is often inferred to mean 'earth ground' when in fact that is not always so, especially WRT antennas.

 

I think you're talking about "Ground Plane" not ground rod
ground plane is the relationship that exists between an antenna and another object such as a vehicle.
The length of the antenna has to be trimmed or adjusted in proportion to the size of the vehicle to create the matching impedance of the transmitter (50Ω)

In a land radio installation, a mobile antenna can be used as long as a ground plane kit is fitted as shown in this photo below

 

When I installed receiving antennas, we used a 6' copper coated steel ground rod, but that has nothing to do with your request. You aren't looking to disipate static electricity from atmospherics, you're looking for a backplane for your transmitting antenna. Using a van will work because of the huge metal top. The pole method mentioned in posts 6 and 9 is a workable method. These guys are getting it right for you. You should click on the "Thanks" button a few times.

 

Thanks.

I got a general picture now.