mobile.exe

[N5VSB]

Due to some mechanical problems with my set up I have been planning or changing my antenna setup on my pickup.  The current setup is off the bed rail and mounted 5 ft off the ground which only leaves me about 7.5 ft of antenna length.

I have used mobile.exe to computer several different scenarios.  Ideally we would like to have the antenna as high on the truck as possible, but using as long of an antenna as possible.  Due to height restriction imposed by trees, we can’t have both of these.  Mobile.exe show the longer antenna wins out over higher placement, but only slightly.  I also know that mobile.exe does not take into account the capacitance coupling to the car body.

Now for the opinion part. Which is better, higher and shorter, or lower and longer?

Jimmy N5VSB

[K0BG]

There is more to it than just height and length, although they play a major part.

Raising the antenna up lessens the coupling to both the vehicle, and the surface (ground as it were). The lessens the APPARENT ground loss.

Increasing the antenna's length increases its radiation resistance. To a point, doubling the length increases the radiation resistance by a factor of four. In other words, increasing the length by 50%, doubles it's radiation resistance.

The over all efficiency is based on the total ground loss, stray losses, resistive losses (coil and conductor), and the radiation resistance, divided by the radiation resistance. The radiation resistance if effected by the length and frequency, obviously.

The hidden factor is the ground loss. Although the ARRL Handbook and other sources list the loss at 2 to 10 ohms (10 through 80 meters), in reality it can be much more than this. It is rather hard to measure and/or calculate.

In a recent technical article published in QST, Jack Belrose, VE2CV, lists an alternate way to arrive at a mobile antenna's efficiency, and in a round about way to determine the ground loss.

I know this doesn't directly answer the your question, with out all of the other variable factors (coil Q and placement in the antenna as examples), the question cannot be answered with certainty.

Alan, KØBG
www.k0bg.com

[K0BG]

Rereading your post.....

One thing you might look into is the use of a cap hat. One of sufficient size would have the effect of increasing the radiation resistance, while not increasing its length. Tom Rusch, W8JI, has some information on his web site about cap hats that is worth reading. Remember, the end of the antenna is the best place for a cap hat. In any case, the rule of thumb is, at least 2 coils lengths away from the top of the coil.

By the way, the one made by DX Engineering is as you as you can buy.

Alan, KØBG
www.k0bg.com

[N5VSB]

Thanks Alan.  You have confirmed what I feared.  Just have to try it and see if I like it.  I think I will have to go with the practical mount and work from there.  I already have the DX Engineering cap hat.  Maybe a Hi-Q 6-160.  

I believe the ground losses here are terrible.  The radio always works better in other parts of the country.

Jimmy N5VSB