Major 102" whip disappointment

[Zman]

A 102" whip is one touch son of a gun to tune right!! The key is ground. I used to run one on my Pontiac mounted on the trunck but i had the trunk lid grounded.

 

[Marconi]

A 102" whip is one touch son of a gun to tune right!! The key is ground. I used to run one on my Pontiac mounted on the trunck but i had the trunk lid grounded.

I agree Zman, and I also think these raised coil antennas may tend to mitigate the problem on mobiles that have a poor ground.

I think I recall a conversation that I had once with Mr. 55, and Terry told me much the same thing was his goal in his designs, and for sure when it came to his big truck antennas. He said those were specifically designed with some big truck limitations in mind.

I can't specifically vouch for anything, good or bad, regarding the 55 line of antennas, or for Terry, but he said it, so I take it as something to consider, and if my models are close to accurate, and you study them closely...they might show some reasons why, the ground plane is terrible, just like Zman says. And, the problem is you can't tell by looking at the ground plane available on a mobile like you can on a base antenna.

I can duplicate the affect with a model where all I do is make the radials a lot longer, like I suspect happens with a lot of mobiles. See below.

The attached model is a base design 102" whip 1/4 wave radiator with a suitable ground plane that produces an almost perfect match at the feed point. In order to try and duplicate the results of my previous model "Bronco model on Top", I simply added very long radials in the second image to show how similar these two will compare. For me, this implies that the ground plane produced by the Bronco model, is way too long for the 102" whip and thus the resonance goes way low, in the model, and maybe in the real world too.

View attachment 102'' whip with very long radials.pdf

 

[NeoHam]

Thanks for the replies

I have verified the problem is the positioning. I took my 1.1 mag mount around to the back bumper and sure enough the swr went to 5:1. I cannot use the 102 on the van. I may get a mag mount and use only when stationary. Is there some kind of kit I can get to make it a base groundplane?

 

[Marconi]

I have verified the problem is the positioning. I took my 1.1 mag mount around to the back bumper and sure enough the swr went to 5:1. I cannot use the 102 on the van. I may get a mag mount and use only when stationary. Is there some kind of kit I can get to make it a base groundplane?

I make an base antenna using several 102" whips, a mobile mirror mount, and a radial hub from an A99, GPK. Just mount the mount and the hub on a short mast and then I mount that on a push up pole. Not only does it produce a great match, it works remarkably well at about 32' - 42' feet high.

Here are a couple of pictures:





This version uses another pipe along side the mast and under the feed point that is about 8.5' feet long. I've used both RG8x and RG8/U as a 8.5' jumper inside the the side pipe, and use a barrel connector to connect to the feed line. I then hose clamp the barrel connector to the mast, and that is supposed to make a bazooka balun action to help prevent current on the feed line.

I think this might tend to help prevent common mode currents, but I really haven't been able to tell any difference in performance, so you might not need the added 1/4 wave pipe.

 

[W5LZ]

To use a mobile antenna for a base station only requires that you provide the 'other half' of that antenna that the vehicle usually provides. That 'other half' can be in several forms, radials are quite common for that. Those radials can be 102" whips or just plain old wire, it isn't going to make much electrical difference, only a mechanical difference.
The amount of 'droop' in those radial does make an electrical difference, they can make the typical input impedance closer to 50 ohms by how much 'droop' there is. How much droop should there be? There's no 'set' answer to that, it depends on how/where the antenna is mounted and what's around it. Vary that 'droop' till you find the best match, that simple.
Have fun.
- 'Doc