height of antenna

[tbharper]

ive got my antenna 25 ft up in the air i was wanting to know if this is high enough or is it to high? someone told me for the 400 mhz range that it should be lower. can anyone help

 

[iMONITOR]

ive got my antenna 25 ft up in the air i was wanting to know if this is high enough or is it to high? someone told me for the 400 mhz range that it should be lower. can anyone help

At 400MHz, it's less critical than 800MHz, but the principals are about the same. What you want to strive for is an unobstructed line of sight path between your antenna, and the antenna(s) of the systems you want to monitor.

How is your reception? If it works fine, don't worry about it. 25' on the average should yield good results in most cases. As for being too high, absolutely not.

Here is a link that helps illustrate this:

http://images.google.com/imgres?imgurl=http://www.21best.com/21_best/electronic/security/wireless/FAQ/FAQ_img/FIG1.gif&imgrefurl=http://www.21best.com/21_best/electronic/security/wireless/FAQ/range1.html&h=198&w=288&sz=7&hl=en&start=11&tbnid=us7qdljNjM-HuM:&tbnh=79&tbnw=115&prev=/images%3Fq%3Dantenna%2Bline%2Bof%2Bsight%26svnum%3D10%26hl%3Den%26lr%3D

 

[prcguy]

A friend of mine recently did some calculations for antenna height vs feedline loss for VHF/UHF. It turns out that over very flat ground (farmers field or water) every time you double the antenna height you get about 6dB of gain out at the horizon. Urban areas and hills give less predictable results. With whatever else my friend used in his calculation, it shows it is always desirable to increase antenna height up to the point where the feedline loss get to about 8dB. There was an exact number, but I forgot. This holds true for basically any type antenna at VHF/HUF and higher and any type feedline. Using 850MHz as a worst case frequency for most scanner folks, this would put you at about 55ft using RG-58 and about 200ft for 9913 assuming the scanner was on the ground and the total coax length was straight up. Going higher than these examples starts to degrade performance in general terms. If your already on a 1,000ft hill then raising your 25ft antenna to 50ft doesn’t follow the same rules.
prcguy