Antenna Height - TowerTalk Forum at eHam.net

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Antenna Height	Reply
by KC2RKU on October 28, 2009	Mail this to a friend!
I am putting up a tower. It is a crank up and it goes between 20-65 feet. I am installing a Mosley 53m, a Hy-grade 6 mt long Jon and a 160 Carolina windom. My question is: Is there a major advantage between 45 feet and 65 ft ? The reason is that at 65 ft there is a posibility of higher winds due to the tree line. Thanks in advance. Skip NC2T

RE: Antenna Height	Reply
by KL7AJ on October 28, 2009	Mail this to a friend!
There are too many variables to answer this question accurately....local ground conductivity...what bands you're operating, etc. However, I would say under MOST conditions, the difference would be negligible. Obviously if your antenna blows over it will have 0 height, so I'd opt for the lower option. :) eric

RE: Antenna Height	Reply
by WB2WIK on October 28, 2009	Mail this to a friend!
Depends what's around the tower. If going to 65' allows your antennas to clear a tree canopy that it would be "below" at 45', you can almost bet the antennas will work better at the higher elevation, especially for VHF. That depends a lot on the kinds of trees you have and how dense they are. In some places (like my own neighborhood) at 65' the antennas all clear power and HT lines, telephone lines, cable TV lines and everything else while at 45' the antennas would be looking directly into (parallel to, and about the same height as) those lines. As such, they all work better above the lines than they do at the same elevation, as some of the lines are pretty close and the RF couples to them. In this situation, not only do the antennas work better at the higher elevation, but the transmitters connected to them cause less interference! If it's a crank-up, why not just "try it" a few dozen times on the various bands at both heights and see what you think?

RE: Antenna Height	Reply
by N3OX on October 28, 2009	Mail this to a friend!
If you're not overloading it it shouldn't be a problem to go to 65. I'd pick 65 feet and clearing the trees over 45 feet any day. I'm not sure the difference is "negligible" on HF. For a 20m beam, (where ground losses don't matter in either case) difference is going to be nearly 2.5dB at very low angles ignoring the effect of trees (which should be weak on HF). While that doesn't seem like much, people seem to be willing to upgrade amplifiers that will do 800-900W PEP to ones that will do 1500W PEP and spend a bunch of money in the process. That's a similar improvement as going out and cranking up to max height. And if you do VHF the difference, as Steve points out, could be profound. 73 Dan

RE: Antenna Height	Reply
by KC2RKU on October 28, 2009	Mail this to a friend!
Thanks for the quick come backs. The trees (hard woods) are in one direction, It looks like the higher I go, the wind is stronger. I live on a side of a small hill/mountain and the way the tree are it causes wind to flow up quickly. Only at certain times. Most will be HF and 6 meter. Thanks again Skip NC2T

RE: Antenna Height	Reply
by WB5JEO on October 28, 2009	Mail this to a friend!
About all you can really say is that great height is not necessarily better. I doubt you see a routine significant difference overall, but you could well find one or the other better for a particular propagation path, if the optimum height for that path happened to be just below 45 or higher than 65. It's going to take a fair amount of experience at both heights to find out.

RE: Antenna Height	Reply
by G3TXQ on October 28, 2009	Mail this to a friend!
Take a look at the chart in Section 2 here on my web site: http://www.karinya.net/g3txq/hexbeam/height_2/ It confirms Dan's figure of a 2.5dB improvement on 20m long-haul paths. The increase in height is also worth 2dB on 10m long-haul paths. But note the penalty of -3.5dB on 10m short-haul paths. The chart indicates pretty clearly why it's nice to have a choice of heights, depending on band and arrival-angle, but not many of us have that luxury :) 73, Steve G3TXQ

RE: Antenna Height	Reply
by WX7G on October 29, 2009	Mail this to a friend!
Skip, the ground conductivity is not very important for a horizontally polarized antenna. At 14 MHz the difference in gain for a change in height from 45' to 65', at a take-off-angle of 7 degrees (DX!), is 1 dB. At 28 MHz the difference it is 1.5 dB. Where it might matter is with state side work where a particular station might fall in an elevation null. At 28 MHz the first null at 45' is 23 degrees while at 65' it is 17 degrees.

RE: Antenna Height	Reply
by G3TXQ on October 29, 2009	Mail this to a friend!
>> WX7G wrote: At 14 MHz the difference in gain for a change in height from 45' to 65', at a take-off-angle of 7 degrees (DX!), is 1 dB.<< I wonder how you arrived at that figure? Dan and I both made it over 2dB difference. 73, Steve G3TXQ

RE: Antenna Height	Reply
by K4SAV on October 30, 2009	Mail this to a friend!
Any gain number difference you calculate is unlikely to be close. He said he lives on the side of a mountain. The results will be significantly different from when an antenna is over flat ground. I wouldn't even attempt a guess at typical answer. On HF, usually the gain goes up with increased height for low angles, but in mountain situations there are cases where it can go down, or produce no change at all. It depends on the surrounding terrain, direction, elevation angle, and frequency. One thing you can say is that the results will be significantly different for different directions. The only way to get an estimate of the difference is to download the local terrain data from the USGS website, run that thru MicroDem to get terrain profiles, and use those in HFTA for the estimate. Jerry, K4SAV

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