Antenna Height Compared to Length

[TDM827]

So I am trying to get a handle on the balance between vertical antenna length and height. It's understood getting a UHF antenna higher into the air is better, line of sight is king.

What's not so clear is, do the lengths of GMRS antennas matter if antennas of different lengths are placed at a height resulting in the top of both measuring an equal distance from the ground. This would be with both antennas used on a 25W radio. 

In other words, will I see advantage with a 10' vertical antenna with a mast and antenna length combination resulting in the top of the antenna being 45' off the ground over a..... 4' vertical antenna with a mast and antenna length combination also resulting in the top of the antenna being 45' off the ground.

 

Lastly, while researching antennas it seems like gain increases with length, but I am not sure if that's a rule.

 

If it's true that antenna gain increases with length, should I be more interested the best gain for my topography, rolling and occasional tall hills in a rural agricultural area, over the length of the antenna, again with both being an equal distance from the ground.

 

I am all for using repeaters, but for this project my main interest is in local simplex coverage. Any guidance would be most appreciated before I put down money on a base antenna.

 

[mbrun]

Good Day TDM.

 

In a lab environment, when they measure an antenna to derive its radiation pattern, they reference the center of the radiating element(s). So it would seem logical that if you were doing scientific experiments with antenna and wanted to compare the coverage of two antennas in real-world space, you too would reference (or mount) both antennas at the same elevation, using the center of the radiating element as the reference elevation. For simplicity, let’s say the smaller antenna had 2 foot radiating element and the larger had 10’. If you mounted the 2 foot antenna at 45’ AGL to center of radiator and wanted to contrast it against the other, you would mount the 10’ at 45’ AGL to center it its radiator.

 

For the sake of a fictitious example. Assuming that the gain of the 2’ and 10’ were both the same (they would not be), the power used was the same, and both were mounted at 45’ to the center of the radiator, I would expect the shorter antenna to win the range contest because its center of radiation is higher above the horizon. Whereas if you compared them both by mounting the bottom of the radiator at 45’, now suddenly the 10’ would win because its center of radiation is higher than the smaller.

 

Now let’s switch to reality. For GMRS, the longer antennas have more gain because they are in-fact some form of collinear antenna design. They are actually comprised of an array of multiple antenna stacked one on top of the other. This has the effect of increasing the effective antenna gain (squashing the donut as some would say).

 

So, if you have an antenna mast and it is 45’ off the ground and achieve range ‘X’ when using a simple ground plane antenna, you will achieve range ‘Y’ when using the 10’ collinear antenna. ‘Y’ will be greater for two reasons. First, it is a higher gain antenna. Second, because the center of radiation is higher.

 

If you live in a valley and need to communicate with radios on the ground in the valley as well as others high on the nearby hills/mountains then a lower gain antenna is usually better. If the terrain is mostly flat to rolling, than a higher gain antenna becomes a candidate.

 

One finally thought. If you live in an area with lots of trees (as I do), a low gain antenna mounted well above the trees is probable to substantially out perform a high gain antenna located below the tree line. So while we say that height is everything, which is true, what is is really all about is getting the antenna above all the obstacles (trees, buildings, earth, etc...) that reduce its range.

 

Good luck on your project.

 

 

Michael

WRHS965

KE8PLM

[OldRadioGuy]

Here's a simple explanation.

 

Longer antennas (properly designed of course) focus your signal so it's concentrated towards the earth's surface.

We're not talking to airplanes or other planets so why waste signal up there.

We want to focus our signal at ground level (or horizon) which is where our contacts are.

 

The gain of the antenna tells you how much it concentrates your signal where we want it.... in this case the horizon.

 

So gain (length) and height both help.

 

You may also need to consider the loss in your coax. 

You may want to run heavier and lower loss coax for the longer run then adapt to something more flexible for the last 10' or so.

I would try and keep coax loss down around 2dB or less if you can.  Less than 1dB would be nice.

 

Vince

[TDM827]

Vince, Michael,

 

Thanks for the help on this topic. You really cleared some things up for me.

 

Given the project goals (strong emphasis on local simplex operation yet repeater capable) and the local terrain, I am leaning heavily toward a mid-gain (4-7db) antenna between 3' and 6' long. Planning on bumping it up a bit so it's center is about 50' off the ground.

 

If I can keep the cable run under 50' from our second floor up to the antenna I will likely go with Times microwave LMR-400 connected with the best quality connectors I can afford. If its over 50' I may upgrade the cable.

 

So far likely antenna candidates are the diamond X50a, Comet GP-3 and Laird CRX450. But I need to do a bit more research on each as it seems like each have certain advantages.

 

Thanks again

[mbrun]

You are making a wise decision on the height. You will be glad you did.

 

Regards,

 

Vince, Michael,

 

Thanks for the help on this topic. You really cleared some things up for me.

 

Given the project goals (strong emphasis on local simplex operation yet repeater capable) and the local terrain, I am leaning heavily toward a mid-gain (4-7db) antenna between 3' and 6' long. Planning on bumping it up a bit so it's center is about 50' off the ground.

 

If I can keep the cable run under 50' from our second floor up to the antenna I will likely go with Times microwave LMR-400 connected with the best quality connectors I can afford. If its over 50' I may upgrade the cable.

 

So far likely antenna candidates are the diamond X50a, Comet GP-3 and Laird CRX450. But I need to do a bit more research on each as it seems like each have certain advantages.

 

Thanks again

 

Michael

WRHS965

KE8PLM