SWR tuning necessary?

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Mar 8, 2024 | 04:39 PM

#11

Rusty Springs

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From: Texas

@KK0G nice Zed page. 73 from AG5CK

Mar 9, 2024 | 05:28 AM

#12

Dsm Limited

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From: Des moines IA

I see a 2 sided problem with an SWR check on the bike.

1st, the CB antenna connectors on the bike are not standard PL259 connectors, that typically are used with CB and SWR meters. Adaptors would be needed going in and out of the meter.

2nd, the antennas on the bike are not "tunable". There is no set screw or adjustment. The only way to tune the antenna, is to trim off small increments, which is fine if shortening the antenna is what is needed. If lengthening the antenna is required, you're SOL.

IIRC, the best way to get all the performance out of the bike CB, is to make sure you have a good ground to the metal plate in the bottom of the tourpack. This is what makes up your ground plane. Unlike cars and trucks, where the whole vehicle makes up the ground plane.

Mar 11, 2024 | 08:27 AM

#13

KK0G

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From: Central Iowa

Quote:

Originally Posted by handirifle

Ok, ever heard it put like that. Everyone seems to to warn us about ruining the radio, it tune is way off. Didn't realize the size of the radio made a difference. I assumed part of its small size, is a lack of a power source, since it probably relies on the bike for its power source.

Radio size has nothing to do with it. The subject is full of myths, misconceptions, and old wives tales and SWR has been the holy grail of antenna performance for decades when in reality it is generally a very minor factor. My theory on why SWR has become the poster child of antenna performance is simply the fact that it is easily measured with little technical knowledge using inexpensive equipment. Antenna theory is a massively huge subject and I'm far from an expert on it but to try to give the simplest answer; the fact that your antenna is so short in length combined with it being mounted to a very small ground plane (your motorcycle) makes it extremely inefficient. Obviously there's nothing you can do about that, it's a small antenna mounted to a small vehicle, it is what it is and no amount of antenna tuning will change that fact. As to why it won't damage your radio, now we're getting pretty deep into forward and reflected power, impedance, wavelengths, harmonics and whole host of other subjects so let's just say that the inefficient antenna pretty much acts like a dummy load and the whopping 4 watts of power your transmitter is capable of producing will quite happily dissipate into it for hours and hours on end with no complaint.

Mar 11, 2024 | 11:31 AM

#14

handirifle

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From: Kalifornia

Well its good to know size doesn't matter, lol. From what most are saying, it seems like making sure its all well grounded, and run with it.

So if thats the case, I will get a second unit to have comms with the wife when in the car an go with that. My Sena headset is lucky to get 1/4 mile range, so if I get a mile or two from this, thats better.

Mar 11, 2024 | 11:54 AM

#15

KK0G

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From: Central Iowa

Quote:

Originally Posted by handirifle

Who said size doesn't matter?? LOL

"Well grounded" is yet another misconception for the reason that most people aren't aware that DC ground and RF ground are two completely different animals but were really starting to go down the rabbit hole so suffice it to say just mount the antenna and radio, pick a channel, and use it. You'll easily get a couple miles of usable range under most circumstances.

Mar 11, 2024 | 11:57 AM

#16

downzero

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Quote:

Originally Posted by Dsm Limited

IIRC, the best way to get all the performance out of the bike CB, is to make sure you have a good ground to the metal plate in the bottom of the tourpack. This is what makes up your ground plane. Unlike cars and trucks, where the whole vehicle makes up the ground plane.

A ground plane is not the same thing as an electrical connection. Electrically connecting your antenna's base to the tour pack ground plate does not make a ground plane.

A ground plane is better thought of as a conductive, reflective sheet. Imagine a vertical antenna with a sheet of copper lining the ground for the distance of the vertical's height in all directions. Yes, the braid of the coax connects to it, but it is the proximity to the vertical and its electrical conductivity that creates that effect, not the electrical connection.\

With cars, the ground plane will be the roof under a roof mounted antenna or the hood next to a fender mounted antenna, not the "whole car."

Back on the original topic, I highly doubt the factory antenna is going to have any measurable mismatch because Harley-Davidson knows exactly where and on what vehicle that antenna is mounted, so it is very likely that the SWR is going to be as perfect a match as is practical for the antenna. Keep in mind also that the wavelength of a CB signal is over 30 feet, so any antenna that is a foot or two feet long is an extreme compromise on those frequencies; optimal performance is far from the case in these circumstances.

Last edited by downzero; Mar 11, 2024 at 12:03 PM.

Mar 11, 2024 | 12:06 PM

#17

handirifle

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From: Kalifornia

I believe the antenna is 19" long. Its not stubby, but not as longs as amfm ant either.

Thanks for the info on ground plane. Makes sense now.

Mar 11, 2024 | 12:52 PM

#18

downzero

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Quote:

Originally Posted by handirifle

I believe the antenna is 19" long. Its not stubby, but not as longs as amfm ant either.

Thanks for the info on ground plane. Makes sense now.

19" long is tiny with regard to the frequencies we're talking about. A 1/4 wave whip (the standard for a ground plane antenna) for CB is 102" long, over 5 times the length of the antenna in question.

I think we all wish that the CB band was where the FRS frequencies are, on UHF, where a similar antenna for that frequency would be about 7 inches long. But unfortunately, when the CB band was created, UHF radios cost a fortune. Hopefully one day there is some political will to create a new citizens band in the UHF area where antennas that fit better on cars and motorcycles, because having to shrink a 102" long antenna to 19" is never going to result in good performance. The reality is that you're making the best of an antenna situation that is 5% or less efficient whereas a proper ground plane antenna could do far better than that, even attached to a motorcycle.