Firestick-II cap raising SWR

[Comanche Scott]

Wanted to check SWR, and output after installing the tire carrier with basket (a lot of metal near the antenna). Had a really weird issue that I've not seen before.
Was able to tune antenna to about 1.3 SWR, but as soon as I put the antenna tip cap back on it would jump up to almost 2.0.
Tried with the cap barely on, and then with it pushed down. Shortened the cap about 1/4", but made no difference.
Used some Plasti-Dip over the tip adjustment, which raised the SWR to 1.5, but better than the cap.
Antenna is a 2' Firestick-II with adjustable tip.

I'm thinking there is something conductive on the inside surface of the cap. I've got a megohmmeter I can check it with. Just thought it was weird, as when new, the cap made no significant ,difference to SWR.
The 4' Firefly made no difference cap on or off. It's a smaller diameter cap, or I'd have tried it on the Firestick.

Any body ever seen this before?
I really like this antenna when on the trail, as it has great output, and sits about even with the top of the Jeep.

 

[Airmapper]

... sits about even with the top of the Jeep.

That is most of your problem.

Anything not ABOVE the Jeep, is reflecting back to the antenna.

You have about the most difficult setup you could possibly have for a CB setup. Short antenna, mounted low. The fact you got it down to 1.5, is admirable.

 

[PhulesAU]

As long as it is above the tub line, you can scratch that off of your list. The biggest problem is that toy antenna, and a bad mounting place. can you return the antenna for at least a 3 footer? A 2' antenna is really just a dummy load.

 

[Comanche Scott]

Thanks guys. :beer:

I have a 4' Firefly that stays in the Jeep, if I need to reach out further. This 2' Firestick-II antenna is great for it's intended purpose. It gets out clear for 2+ miles depending on topography, and with an SWR at 1.5 it is plenty good enough for the little I talk. It's a great trail antenna.

Just strange the the tip cap alone would make a 0.7 jump in SWR. That's pretty big for less than 5 watts output. I'll figure it out next time i grab the M-O-M. I'm betting there is something conductive coating the inside of the cap.

 

[4x4junkie]

I see no reasons whatsoever to ever have such a tiny little dummy loa... er, antenna on a vehicle.

Put your 4' Firefly back on and leave it alone. If you think it's too tall because of low trees, then put a spring on it. The spring will let it flex harmlessly out of the way should a branch or overhang hit it.

 

[Comanche Scott]

Thanks 4x4 Junkie, :beer:
The 2' antenna works great on the trail. I really like it. The radio is very clean on Tx, and reception is excellent.
4' Firefly is an awesome antenna, and if I need to get out more than a few miles, it goes on. But so far I haven't "needed" it.

Just had a question on whether anyone has seen an issue with the tip cap.
I checked the cap last night with the megohmmeter. As I suspected, there was something conductive coating the inside of the cap.

 

[BigSwede]

It has been a long time since I SWRed my Firestik II, but I do remember the cap changing the number. Can't recall how much though.

 

[Comanche Scott]

Thanks Steve,
Great input! :beer:

 

[JGBB04]

Always set swr w/ the cap on

http://www.firestik.com/Tech_Docs/Setting_SWR.htm

 

[Comanche Scott]

great input in this thread! Thanks again to all who posted!

Always set swr w/ the cap on

http://www.firestik.com/Tech_Docs/Setting_SWR.htm

:iagree:

All the input here has been very good, and also (imo) very valid.
Thanks again guys! :beer:

 

[Utah KJ]

Yeah, you need to leave the cap on when you're checking SWR. It says so in those instructions you tossed. :costumed-smiley-007

 

[Comanche Scott]

Yeah, you need to leave the cap on when you're checking SWR. It says so in those instructions you tossed. :costumed-smiley-007

You were here when I installed and tuned this system? And didn't even offer to share the reefer?

:elkgrin:

 

[DaveInDenver]

The cap must apparently be a dielectric and is in effect like putting a capacitor at the end of the antenna. It's like a radome or capacitance hat.

Anyway, tune the antenna so that it dips without the cap and note the frequency of lowest SWR. For the the sake of argument say it's 1.5:1 SWR at 27.00 MHz.

Put it back on and re-sweep it. At your frequency of interest the SWR will go up (say to 3:1) but it will have a new dip somewhere higher or lower. Say for this example it ends up with 1.5:1 SWR at 27.50 MHz. I do not know if it'll go up or down and how much, just an example.

Now you know the effect of the dielectric load for the cap. Take it back off and tune the antenna so that your SWR dips to 1.5:1 at 26.50 MHz with the cap removed and you should see 1.5:1 SWR at 27.00 MHz when you put it back on. It should be 3:1 SWR now at 26.50 MHz, though.

The actual offset might not be exactly the same at all frequencies but you should be in the ball park.