General SWR Problems

[aldaman]

I just bought a cobra 29 ltd to go into my 1988 Cadillac Elderado. I am using a cigarette lighter power source until I get more a more permanent power source. I am using a K30 antenna mounted in the center of the roof and a radioshak swr meter. My problem is that on channel 1 and 40 the swr in a little above 2, but only 1.1 on 20. I didn't think that swr could be that low on this kind of set-up and was wondering why the low point was at 20? What if anything should I be worried about?

 

[Robb]

That is typical of what I'd seen with the K30 antenna too. Not the sharpest piece of work. The best and only way to solve this problem - sorry to say - is to purchase a better antenna.

A Wison 1000 mag-mount antenna is vastly superior and would be highly recommended.

 

[chale]

you are experiencing what everyone with a K30 experiences...Narrow Bandwidth...just the nature of the beast

if you talk on 19 all the time...use it, if you plan on talking say below channel 10 or so or above channel 30...i'd consider the Wilson as previously mentioned

 

[eagle1911]

I've never used the K30.. only the K40 (which, oddly enough, I hear very little about these days) which used to be pretty amazing.. I worked many different countries with a K40 and a stock Galaxy 88 in my ride.

It just sounds to me like the K30 has a narrower bandwidth than most people would like.. I doubt the issue is with your installation.

If you look at an SWR plot of an antenna you see that there is a curve, with the lowest point being at the middle of the band, and the highest points being on the edges of the band.

In this image, replace the frequencies shown with CB channels, with 40 and 1 at the edges and 20 in the middle and you'll get an idea of what I mean:

The reason is that the antenna is really only resonant at one frequency, in your case right around channel 20. As you move away from that point the match between the antenna and radio changes, increasing the SWR. This is perfectly normal, and all antennas exhibit different curves depending on their design.

 

[chale]

I've never used the K30.. only the K40 (which, oddly enough, I hear very little about these days) which used to be pretty amazing.. I worked many different countries with a K40 and a stock Galaxy 88 in my ride.

It just sounds to me like the K30 has a narrower bandwidth than most people would like.. I doubt the issue is with your installation.

the reason you don't hear about the K40 is because the Wilsons beat the pants off them and offer more power handling capability, the only competitor to the K30 is the lil wil and i'll take a K30 over the lil wil, but both are extremely narrowbanded

 

[eagle1911]

Pretty sad.. My experience was over a decade ago, and I used both the Wilson 1000 and the K40.. I LOVED the K40.. the Wilson was good too, but I really liked the K40 more.

 

[RatsoW8]

Mag mount antennas are the least efficient of any antenna you can use for HF but, if you don't want to or can't drill a hole your stuck with the problems of a mag mount.

 

[aldaman]

thanks everyone for the reply. I think that about explains it I guess I gotta get a new antenna. Is there any explanation for the swr being so low at 20 though? I didn't think even with a narrow frequency range that it should be that low.

 

[eagle1911]

A low SWR is never anything to worry about. The lower the better. As you can see in the plot above, the low point should ideally be in the middle of the band. Channel 20 is the middle of the Citizen's band. All is as it should be.

 

[GnG8d]

Channel 20 is the middle of the Citizen's band.

It's 19 if you want to get technical :tongue:

 

[hotrod]

ive owned dozens of mg mount antennas. the k30 was the worst
i liked the lil wil better performance wasnt much better but swr seemed
to be a tad better. if my memory is right.if you can use a longer antenna
the look at the wilson 500 @ 4ft [about] or wilson 1000 @5 ft [about]
the 500 surprised me it worked almost as good as the 1000 and was
shorter.the downfall is you cant remove the coil./whip .gotta remove whole antenna
on the 1000 you can remove the coil by unscrewing it keeping the mount /coax
in place.

 

[Beetle]

The reason is that the antenna is really only resonant at one frequency, in your case right around channel 20. As you move away from that point the match between the antenna and radio changes, increasing the SWR. This is perfectly normal, and all antennas exhibit different curves depending on their design.

Actually, the mismatch to be concerned about is the one between the feedline and the antenna itself. That's where "SWR" is really determined.

 

[eagle1911]

You're absolutely right, lousy wording on my part. What the meter sees has nothing to do with the radio technically, it's all about the feedline and the antenna. The radio just senses the mismatch, if there is one.