CB Antenna, High SWR's

[Strõkër~X]

what do the SWR's do?

[JBakerFX4]

Standing Wave Ratio. Determines if your cb system actually "works" or not. Makes the difference between talking miles and talking 75'. Determines how long of a life your CB radio is going to live also

[geekdownrange]

Originally Posted by Strõkër~X

what do the SWR's do?

Page full of text and math. The wikipedia entry was even more math-babble intensive.

SWR (Standing Wave Ratio) gives you an easy to understand ratio of how much of your power is being radiated by the antenna and how much is reflecting back to heat up your coax and possibly damage your radio.

Maximum power transfer from source to load occurs when impedances are matched. Almost all radio transceivers are designed for a 50 ohm load. If the antenna presents a 50 ohm load, maximum power transfer will occur. That means your 4W (~12W PEP SSB) from your CB is almost all being radiated by the antenna. It is a little more complex with things such as cable loss, complex impedances, etc - for the moment lets just assume that if we match everything up 50 ohm transmitter to 50 ohm load that we're exactly where we want to be.

SWR examples - If we hook up our 50 ohm CB to a 75 ohm load we'll have an SWR of 1.5:1 (75/50). If we hook up that same CB to a 25 ohm load we'll have and SWR of 2:1 (50/25). By convention, the larger number is used for the numerator.

Behind all the math and RF-geekiness we can boil is down to some simple rules of thumb:

1:1 - great, nifty, and almost unachievable without extra autotuner hardware (or your transmitting into a 50 ohm resistor)

1.5:1 - a very good target. This is usually achievable.

2:1 - acceptable

>2:1 - figure out what is wrong because you may damage the output transistors of your CB, or at the very minimum a professional transceiver will turn down the output power to prevent damage.

-Brian
(Yeah, I really do this for a living.)

[JBakerFX4]

brian- so If I disconnected the Coax from the radio, and put a Ohm meter on the coax center to shield... should it read 50 ohms? or close?

[geekdownrange]

Originally Posted by JBakerFX4

brian- so If I disconnected the Coax from the radio, and put a Ohm meter on the coax center to shield... should it read 50 ohms? or close?

Grr.. I wrote a long reply and the bit-gremlins ate it.

Unfortunately, DC resistance and AC impedance are very different beasties. The average mobile CB antenna will test as an open on a VOM. You can also build antennas which would show a short at DC on a VOM (good for static dissipation).

[JBakerFX4]

well geek, the best I could get my SWR's on the behind cab stake hole mount was 1.4/1.9 with 3 ground straps and 1 ground wire to the mount. I put the 4' francis on there with spring.... WOW! SWR's went SKY high. 2.5/2.9, pushing 3. That location just sucks. I pulled the stake hole mount off and am going with a wilson 5k mag mount. Have it run into my permanently mounted mini8 coax. Seems to be my only option.

[FLYROD72]

I have a K 40 trunk mount. Made a bracket to bolt to the fender and it works great. On ch 1 neddle does not move and on 40 it just barely moves the width of the needle. I like the K 40 because the end that screws to the back of the radio comes off to make it easier to run through the fire wall. With your ant. close to the cab it will kill your swrs. Just my 2 cents worth.

[geekdownrange]

Originally Posted by FLYROD72

I have a K 40 trunk mount. Made a bracket to bolt to the fender and it works great. On ch 1 neddle does not move and on 40 it just barely moves the width of the needle. I like the K 40 because the end that screws to the back of the radio comes off to make it easier to run through the fire wall. With your ant. close to the cab it will kill your swrs. Just my 2 cents worth.

It is very true that you don't want higher metal objects close to the antenna. Ideally the base of the antenna would be higher than any metal object within a few wavelengths. Since a wavelength at CB frequencies is 35-40ft that may be impractical.

It is also important to remember that a wide SWR bandwidth isn't always a good thing. A good quality 50ohm resistor of appropriate wattage across the end of your coax will give you a a nice flat 1:1 SWR over a wide frequency range. The CB band is relatively narrow given its frequency so it isn't typically a huge issue.

[mrnecsteve]

The liklihood of ever getting the swr's low,while being close to the side of the cab is pretty low. If you are going to do a permanent mount,i recommend you consider an
NMO mount....it will then be interchangeable with differant band antennas that have standardised the use of NMO mounts.Visit a ham shop and ask for 11 meter setups and advise. There are times when i switch between 1/2 wave and quarter wave ....a 1 minute task to switch or or simply remove for clearance.

[JBakerFX4]

Got my wilson 5K mag mount all set up nicely. 1.1 on ch 20, 1.3 on 1 and 40. Not bad at all. Mounted the mag on the roof of the truck, checked swr's every 2". 24" foward of the brake light worked best. I can talk across town easily barefoot.