Tuning a Francis antenna?

I agree, thats the way I usually install my radios.

 

You can tune a Francis, just remove the end cap and dig around in the fiberglass for the wire. They are usually just a bit "long". Same thing for the firestik I.

I like firestik because they are more "top loaded", but they are thick and heavy, and tend to break or tear up/stress your mounts 5' or more. Francis is much lighter but I wonder how they are wound.

I watched a video on YT about Francis. Just an opinion that they didn't perform as well but we're more convenient because they were "pre-tuned". I don't agree with that. Francis used to make a 102. It had the same performance as a steel 102... maybe even better as it didn't flop around as much when driving around. I think they get a bum rap from r no good reason. Maybe it was thier marketing choice of wording...

 

I like the Francis antenna on a high mirror mount. Especially when I had a Granite Mack and a 389. They worked great off road with the trees and such. Always a good idea to put some blue Loc-tite in the cap after you tune to keep the trees from snatching them off. But, I do get out better and lower swr with the Skip Shooter's.

 

The 96” Francis CB50
AMAZER

Three-wire each of different length; joined at base

Pre-Barjan (2/2000 acquisition)

 

Ok so on my mobile unit for work I have a Stryker 655 and a 5' adjustable tip firestik how do I figure out what the shortest but best length of coax to use I am going to but RG8X for it soon and I'm looking to keep it the shortest possible because I have 18' now and I have a bunch of extra I have zig zaged acrossed the cab and I would like to get rid of the zig zag process as much as I can so can someone tell me the process on how to figured out how long I need it to be but as short as I can get it to get maxium power and distance possible because I drive dump truck and on well pads it is best to be able to hear and get out as early as possible

 

That depends on your feedpoint impedance. If purely resistive 50 ohms, cable length won't matter.

 

Unfortunately there are many different theories about coax length. But to simply answer your question, just use the formula

492 X VF / Frequency.

The thing with RG8X coax is that the velocity factor ranges anywhere from 75 to 82% from what I’ve seen. So you need to ask your dealer what the VF is when you buy it. For example, if your coax has 80% VF, then using the formula, your coax length for channel 20 would be:

492 x .80 = 393.6 / 27.205 = 14.5’

So if you would cut your coax to 14 1/2 feet in this example. I like to adjust my mobile antenna in the middle of the cb band, that’s why I use channel 20 in my example.