Mounting a CB antenna on a modern hatchback without drilling?

[drewdownkali]

Heh. there antenna chart won't pull up for me and I can't post anymore pictures.

 

[mmckenna]

It actually starts at 47 and says to cut it to 25.5 for 27mHz.

25.5 would be way too short, was that a typo?

The cutting chart from the Laird/Antennex website for that model shows 47 inches to be the correct length for the spring base for 27MHz. I'd really suggest starting there before you do anything else. an inch or so can easily throw it out of resonance and give you a high SWR.

 

[mmckenna]

Yeah, it took me a while to get it to download the chart. I've got it saved as a PDF if you want me to send it to you.

 

[drewdownkali]

Sure you looked up "cb27s" and not cb27 or c27?

 

[mmckenna]

Same antenna. The cutting chart is for the coil. The "B" means it's black, as opposed to chrome. The S means it ships with the spring. The cutting chart I have is for the C27 coil. There is a column for no spring and a column for the spring. The chart shows 47 inches for the C27 coil with the spring.

47 inches is about right for a base loaded CB antenna. I used to use the Larsen brand NMO27, which is basically the same design. 47 inches was the length for those, also. The whips shipped as 49inches long and I always had to cut them down. I always did through hole installs on the roof, so getting the SWR nailed wasn't a problem.

 

[drewdownkali]

Yeah, it took me a while to get it to download the chart. I've got it saved as a PDF if you want me to send it to you.

Okay. There thing isn't working. Didn't know they had pdf also.

Back of bag same as what I read on their site before.

26.750 mhz 47"

27 mhz 45-1/2"

27.5 mhz 43-1/2"

28mhz 42-1/2"

28.5mhz and so on

 

[mmckenna]

OK, there ya go. That should get you in the right area. 49 inches is putting you down probably around 25 MHz or so, and that would explain why the SWR is 3:1.

 

[drewdownkali]

That pdf chart is slightly different, doesnt mention that how the whip comes (at 47") is 26.750mhz as this bag and the web site (non pdf) calculator said.

 

[drewdownkali]

myself and the cb guy could not get it tuned when he had the 47 in whip in it. So he trimmed a bit (very small amount) as the chart says 25.5 whip for 27mhz for spring model.

 

[canav844]

Dielectric grease is an insulator. It does not conduct electricity, so expecting it to ensure a good ground is incorrect.

Interesting, I always thought it was a mild conductor. It does do a really good job of preventing corrosion though, which in my experience with mobile installs, can be half the battle some days.

I have the non-spring version of the antenna, but before I realized the difference when the photos would display; my hope was to measure my setup, and give the OP a ballpark to start, or possibly identify a whip that was too short. I don't remember what the packaging said, but I remember going about 1/2in longer than the recommendation and slowly working down shorter until I got it dialed in. If the whip length is proper then, my next guess would lie in the feed line.

Looking at what you're reporting the packaging to say, I'd probably trim to 46" then 45 1/2 then end up somewhere between 45 and 45 1/4. But the only real way to tell where you're at is comparing the SWRs across the band. As soon as you get under 3:1 on one of the following you should start to be able to troubleshoot accurately.
Ch1 __:1
Ch19__:1
Ch40__:1

 

[mmckenna]

OK, well I'd go off the bag then.
You'd want to (ideally) cut for 27.185, which is channel 19. That frequency is in the center of the band (not channel 20, as you'd think).

Getting it in close should bring it down considerably. Once you have the SWR down, you can do the channel 1 vs channel 40 thing to find if it's too short or too long. At that point, you should be well within acceptable range and you should notice an improvement in performance.

 

[drewdownkali]

https://www.facebook.com/photo.php?...267.1073741830.100000031515732&type=3&theater

this is back of bag

 

[drewdownkali]

This is the one I printed off the site with the box you type you model into.

https://www.facebook.com/photo.php?...267.1073741830.100000031515732&type=3&theater

 

[mmckenna]

Interesting, I always thought it was a mild conductor. It does do a really good job of preventing corrosion though, which in my experience with mobile installs, can be half the battle some days.

Dielectric grease is often packed around electrical connections in situations where water ingress can be a problem. I use it around automotive light bulb contacts to prevent water from getting in there and crapping things up. Really good stuff to use on boat trailer lights. I also pack it in around the trailer connectors. The contacts are "wiping" so they'll clear a path. We also use it on our battery connections at work, to prevent corrosion. We are on the Pacific Ocean and we have a lot of large (1000 amp/hour +) 48 volt power plants for my telecom systems. Expensive batteries and corrosion will spoil my day. Cheap insurance.

 

[mmckenna]

Facebook won't let me see it.

 

[drewdownkali]

hmm. changed privacy settings

 

[drewdownkali]

Think it might have been camera slipping into a different mode. Pics too big.

 

[mmckenna]

Uh, I'd use the one off the website. That doesn't look entirely legit. Could be someone did their own testing and made their own chart, but the actual location of the antenna will change the length needed to make it resonate.
Also, the radiating part of the antenna is what needs to be resonate at the frequency you are using. The coil spring at the base is part of the radiating element, so that needs to be figured into the length. According to the chart I pulled off the website, the spring adds 2 inches, so it sounds like yours may be too long. I'd really use the chart off the website and make sure you use the lenghts for the whip with the spring installed. 2 inches too long is going to throw you off bad enough to keep you up all night.

 

[drewdownkali]

Dumb camera. Easy mode my butt.

 

[drewdownkali]

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