Tuning CB SWR on an untunable antenna


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Tuning CB SWR on an untunable antenna

Post by WingAdmin » Sun Feb 25, 2018 4:53 pm



I spent my day helping a friend with his bike - he was unable to get the SWR to tune on his antenna, no matter what he tried. The SWR was off the scale, if he lengthened or shortened the antenna. The bike had been transmitting a garbled sounding signal to other bikes, and it wasn't receiving well, either. No wonder!

Incidentally, if you are wondering what tuning an antenna entails, and why your CB antenna should be tuned, read the explanation at How to tune your CB antenna SWR.

The antenna in question was a J&M fiberglass whip, intended for mounting on motorcycles with no ground plane.

I too was unable to get it to do anything other than full scale deflection on my SWR meter. I pulled the antenna off the bike and measured the continuity from the base to the wire under the cap at the tip - and there was none! Somewhere along the length of the antenna, the spiral wire inside it had broken.

We pulled the antenna off and replaced it with a new one, but the SWR was still not great - around a 3.5. I pulled the ground off the bottom of the antenna and instead mounted the ground to the metal frame to which the antenna as mounted. That was the trick. Got the SWR down to 1.1 on channel 1 and 1.4 on channel 40.

I had never seen a fiberglass whip with the internal wire broken before - I guess there's a first time for everything!



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Re: Tuning CB SWR on an untunable antenna

Post by ka4yqi » Mon Feb 26, 2018 12:54 am

Now that is unusual. Does it have the standard 3/8 24 thread on the bottom? Have you seen the ones that has a nylon string wound Like a copper wire?

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Re: Tuning CB SWR on an untunable antenna

Post by WingAdmin » Mon Feb 26, 2018 9:30 am

ka4yqi wrote:
Mon Feb 26, 2018 12:54 am
Now that is unusual. Does it have the standard 3/8 24 thread on the bottom? Have you seen the ones that has a nylon string wound Like a copper wire?
Yes, it had a standard thread like you describe. How is the nylon string made conductive to carry the RF?

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Re: Tuning CB SWR on an untunable antenna

Post by AZgl1800 » Mon Feb 26, 2018 10:41 am

WingAdmin wrote:
Mon Feb 26, 2018 9:30 am
ka4yqi wrote:
Mon Feb 26, 2018 12:54 am
Now that is unusual. Does it have the standard 3/8 24 thread on the bottom? Have you seen the ones that has a nylon string wound Like a copper wire?
Yes, it had a standard thread like you describe. How is the nylon string made conductive to carry the RF?
in 40 years of using those things, never had one fail.
so, never took it apart to see what is in it.
I suspect they are using a thin bronze mesh that will flex w/o breaking
~John

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Re: Tuning CB SWR on an untunable antenna

Post by WingAdmin » Mon Feb 26, 2018 11:00 am

AZgl1800 wrote:
Mon Feb 26, 2018 10:41 am
WingAdmin wrote:
Mon Feb 26, 2018 9:30 am
ka4yqi wrote:
Mon Feb 26, 2018 12:54 am
Now that is unusual. Does it have the standard 3/8 24 thread on the bottom? Have you seen the ones that has a nylon string wound Like a copper wire?
Yes, it had a standard thread like you describe. How is the nylon string made conductive to carry the RF?
in 40 years of using those things, never had one fail.
so, never took it apart to see what is in it.
I suspect they are using a thin bronze mesh that will flex w/o breaking
It's a very thin (I didn't measure it, but looks to be 22 or 24 gauge) copper magnet wire (coated with lacquer insulation) wound around the fiberglass pole, and then coated with a flexible plasticized coating. It's a loose spiral up the pole until it gets near the top, where it is wound tightly for a couple inches, to create a load coil. It then spirals loosely again up to the top, where it can be pulled free and trimmed in order to adjust the SWR. So if the antenna was bent too far, or at a specific place repeatedly, the wire could fatigue and break, which is what I suspect happened here.

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Re: Tuning CB SWR on an untunable antenna

Post by ka4yqi » Thu Mar 01, 2018 1:30 am

WingAdmin wrote:
Mon Feb 26, 2018 9:30 am
ka4yqi wrote:
Mon Feb 26, 2018 12:54 am
Now that is unusual. Does it have the standard 3/8 24 thread on the bottom? Have you seen the ones that has a nylon string wound Like a copper wire?
Yes, it had a standard thread like you describe. How is the nylon string made conductive to carry the RF?
Usually they use it when they make the other antenna a am/fm/ cb antenna. Then the antenna is just a dummy so it doesn't look off balance. Just for looks

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Re: Tuning CB SWR on an untunable antenna

Post by SlowTyper » Thu Mar 01, 2018 2:52 am

On my GL1200, there was an antenna combiner in the fairing that affected the SWR. It had a variable capacitor inside that could be adjusted to obtain an optimal SWR. Thus, tuning the antenna itself wasn't important since the cable loss was minimal for such a short length. All that mattered is that the transmitter saw a low SWR so that it would operate efficiently and put out full power. Of course this assumes there actually is an antenna connected, and not a broken wire...

Because of the combiner, it was important to check the SWR at the transmitter cable rather than the antenna cable.

For what it is worth, I later removed the combiner and used separate antennas for the CB and the radio. I did this for looks, rather than performance.

I am guessing it might be helpful to explain what a CB is, for some younger readers. Ha!

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Re: Tuning CB SWR on an untunable antenna

Post by Wayne » Sat Mar 03, 2018 10:13 am

Usually the solder joint at the base of the antenna is where it will loose continuity either from fatigue or just a bad solder connection. We tune hundreds of the antennas for our antenna kits and occasionally find a bad solder connection. The dummy antenna on the Gold Wing with monofilament line is made to balance the looks. Some customers order 2 regular antennas but that just defeats the purpose of the antenna system. With the splitter and the dummy antenna it will give us twice the range front to back instead of over 50% going to the left. Easy way to test this is to just drop the AM/FM and test the front to back range. Think of the OEM antennas setup as a 2 element beam antenna with the AM/FM being a reflector.

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Re: Tuning CB SWR on an untunable antenna

Post by New wing rider owner » Sat Mar 03, 2018 12:12 pm

If you want top performance from the Cb use screen wire under All the snap in panels and the trunk and bags. It must be grounded to the frame and ground of the antenna. As my 93 antennas are set up the FM Antenna would not be a reflector as it doesn't have a center loading coil and is too short to have a lot of effect on the full wave 27 meg of 36 ft. I

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Re: Tuning CB SWR on an untunable antenna

Post by WingAdmin » Tue Mar 06, 2018 9:01 am

Wayne wrote:
Sat Mar 03, 2018 10:13 am
Usually the solder joint at the base of the antenna is where it will loose continuity either from fatigue or just a bad solder connection. We tune hundreds of the antennas for our antenna kits and occasionally find a bad solder connection. The dummy antenna on the Gold Wing with monofilament line is made to balance the looks. Some customers order 2 regular antennas but that just defeats the purpose of the antenna system. With the splitter and the dummy antenna it will give us twice the range front to back instead of over 50% going to the left. Easy way to test this is to just drop the AM/FM and test the front to back range. Think of the OEM antennas setup as a 2 element beam antenna with the AM/FM being a reflector.
This was an antenna that had been used for years and had performed admirably, but last year the signal from it became very garbled, which is what led to trying to figure out what the problem was. I've installed a couple of Sierra antennas on my own bikes, and they have always been tuned 100% perfect right out of the package - and now I know why. :)

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Re: Tuning CB SWR on an untunable antenna

Post by Wayne » Tue Mar 06, 2018 10:58 am

reply to New Wing Rider, if you are familiar with an old yagi tv antenna there is 1 driven element that the cable is connected to and several dead short director and longer reflector elements. As far as the CB antenna is concerned the AM/FM antenna on the G/W is nothing but a reflector of a 2 element yagi. Get a field strength meter and walk around the m/c with the am/fm up and then fold it down and you will see the difference.

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Re: Tuning CB SWR on an untunable antenna

Post by New wing rider owner » Thu Mar 08, 2018 3:26 pm

Finally got the thing to let me write a message. I really don't want to argue with anyone on here. For the am FM Antenna to have effect on the signal A mile or more it would first have to be grounded. Second as a director would have to be a few inches shorter as the electronic wavelength. I have built many antennas from 2 meters to 160 and those that are tunable from 6 to 160. We are in a low sunspot cycle now and should have good communication from bike to bike now for quite a few years to come except for local interferance. May have very short periods of skip at times. Maybe you can put a retreaviable ground wire on your n
Bike like they dropped out of the military planes to transmit to the submarines. Just a thought

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Re: Tuning CB SWR on an untunable antenna

Post by AZgl1800 » Thu Mar 08, 2018 5:17 pm

I totally agree, but was waiting for someone to mention the "electronic facts" of resonant directors and reflectors.

The AM/FM antenna has zilch effect on the CB antenna's radiation pattern. the only effect it will have is a bit of "Capacitive Reactance" that might detune it a bit if it were removed.... but it won't be removed, so the effect of the AM/FM radio antenna is zilch, nada...

Building antennas and tuning them for "best forward gain" is something you never forget. A trifling thing like a short FM radio antenna is not going to have any effect what-so-ever on a CB antenna. The CB antenna "should be" 108 inches long... the highly shortened center loaded whip is very inefficient due to its' overall length, and the lack of a decent ground plane surface.

Skip cycles will make some long distance contacts possible, but don't depend on it. The 1 kilowatt boys out there will be stronger. ( laws about the 5 watt limits don't exist for them )
~John

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Re: Tuning CB SWR on an untunable antenna

Post by gervais » Wed Mar 14, 2018 7:29 pm

well
if i may add,those "AllDress" antenna are just enough to have some rf propagation,nothing more.

And other aspect is "ground effect here" if you fixed the antenna on chrome surface,,wont be extraordinaire and you should take a starp cable and connect it at a good ground of the bike an at the fixation bracket of this antenna.

i wonder how mine is working on my 1984,,,and someone was speaking about an antenna tuner somewhere??? what is the TX power of these cb's? 4 watts.

i just installed an APRS Tracker on my GW with a portable radio,the main problem is ground effect,,speak of that with any installer of Motorola Police Pack on Motorcycles,

Gef

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Re: Tuning CB SWR on an untunable antenna

Post by SlowTyper » Wed Mar 14, 2018 9:02 pm

If my memory serves me correctly, the maximum legal output power for CB radios is 4 watts.

However, the maximum legal input power to the final stage is 5 watts. In many cases, the output circuitry is less than 80% efficient so the output power ends up being less than the legal maximum of 4 watts. For example, if the output stage has an efficiency of 60% than the maximum output power would be 3 watts (5w x 60% = 3w). On the other hand, if the output stage were 90% efficient, then the maximum power feeding the output stage would be legally be limited to 4.44 watts (4.444444w x 90% = 4w legal max).

And while we are on the subject of output power, a comment about SWR might be in order. SWR stands for Standing Wave Ratio. When the antenna is not tuned perfectly, not all the power on the cable goes out the antenna. And what does not go out the antenna bounces back to the transmitter, which in turn bounces it back to the antenna. This signal bouncing back and forth is called a standing wave. The SWR is the ratio of the amount of power in the cable (including what is bouncing back and forth) compared to what is going out the antenna. For example, suppose there is 4 watts going out the antenna and the SWR is 2:1. That would mean there is 8 watts of power traveling the antenna cable (4x x2 = 8w).

A high SWR can be problematic for several reasons. 1) The power traveling the antenna cable can exceed the power rating of the cable. This is not normally a concern with CBs because even an SWR of 25:1 results in only 100 watts max traveling the cable [and much less in actual practice; see next note]. 2) The transmitter output stage has to reflect any power bouncing from the antenna, and send it back toward the antenna. In the case of an SWR of 25:1, this means an output stage designed for 5 watts has to reflect 80 watts! That would definitely fry the CB radio. Consequently, most manufacturers design the radio to reduce the power output any time the SWR exceeds 3:1; some manufacturers design their radios to reduce their power output at much lower SWRs -- perhaps at only a SWR of 1.5:1. For example, let's say the output stage can only handle 15 watts max before it would be damaged, and so the manufacturer designs it to limit the power to 10 watts just to be safe. Let's further presume the stage is 80% efficient and the SWR is 25:1. In this scenario, the radio would not put out 4 watts, but rather only 1/3 of a watt (10w x 80% / 25swr = 0.32w).



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