SWR readings way off

[FubarPerson]

I made a 1/4 wave (102") cb dipole using #12 wire as the elements connected to the coax without a balun or a choke.

I have an old Cobra 29 LTD I tried to use with it. The SWR is between 4:1 and 8:1 and shows only 2 watts out (I know that's very bad). The SWR meter in the radio is not working so i used an external meter. Could that affect the reading being that there are two meters?

I also have a cheap Radio Shack cb I got at a flea market. I hooked it up to the same antenna and the SWR doesn't move at all (1:1) and shows 6 watt output. I doubt there was any mod to it.

Does anyone know what's going on here? I don't want to mess up either radio. Any help would be great.

PS: Both radios are second hand and I've done NO MODS to the radios.

 

[Wire Weasel]

Hey FUBAR,

Sounding like something is wrong with the Cobra if the RS radio is showing normal-good operating output & SWR, but I agree that is unusual. Take the cover off the Cobra and see if anything is touching the wire going from the finals area to the center pin on the coax output.

Good luck

 

[Moleculo]

Did you make a dipole with two 102" sections, or did you make an antenna that has an overall length of 102"?

 

[dudmuck]

I made a 1/4 wave (102") cb dipole

Just to be certain, thats 102" for each side? You need nearly 18 feet of total space for that.
I assume its horizontal? It might need to be slightly longer on each side, maybe 103" for CB.

Make sure you use three insulators in total. one for the middle feedpoint, and one on each end. But for just a barefoot CB, regular non-conductive string might be ok for the ends.

 

[BOOTY MONSTER]

"I made a 1/4 wave (102") cb dipole using #12 wire as the elements connected to the coax without a balun or a choke."

are you using this on a base ? if so it needs a counterpoise/ground radials . i made a starduster copy using 12 gauge wire and pvc pipe and im well below 1.3 with the feedpoint at about 27 feet and i do use a bauln . heres some good info on grounds for base antennas .

Antenna Notes

this is basically what i built using sloped ground radials at about 35-40 degrees .
Antenna Notes

 

[W5LZ]

First, you'll have to clarify what that antenna is, Mole's question. Basically, it means that if you have 102" of antenna connected to the center conductor of the coax and nothing connected to the braid of the coax, then you've only got half of an antenna. If that's the case, expect SWR that's not good, and don't rely on a watt meter's readings.
So until we know something more about your antenna, there's not much sense in trying to figure out what's wrong with it.
- 'Doc

 

[FubarPerson]

I have 102" of wire connected to the center wire for transmit and 102" of wire connected to the braid for receive. It's mounted horizontal about 8' above the ground. I didn't use insulators as I put it together rather quick just to experiment. The center wire and braid are not touching and the elements aren't touching any metal.

I want to use it with a mobile cb as a base for some local use, or if conditions permit, some "skip".

What threw me off was that the Cobra SWR was way high with 2 watts output and the cheap radio shack was perfect with 6 watts on the same antenna??? :blink:

 

[C2]

That is about the right way to make a dipole.

One clarification, both elements are used for transmit and receive.

Make sure you do not have a coax problem (something shorting or open, etc.). Often these kinds of problems are related to bad coax, usually at the connectors.

Also, the Cobra might be fubared. 2nd hand, who knows whats been done. Usually if the radio puts out power, the SWR is not going to change much, so maybe the cobra is off frequency? Hard to say...

 

[FubarPerson]

I've used both in my truck with both a magnetic husler and a 3' fiberglass antenna. Granted I don't get out all that far with those antennas, but people still say I sound good when they do hear me.

I never used a meter with the other antennas. Maybe I should try that......

 

[Highlander_821]

It would maybe help if you can measure the SWR on channel 1, channel 20, and channel 40, and post the results. That info might tell us if trimming the antenna will bring down the SWR.

 

[FubarPerson]

The cobra is 4:1 and up on 1, 20 and 40

the radio shack is 1:1 on 1, 20 and 40

Could the external meter be giving a false reading for the cobra since the cobra has an internal meter?

Remember the internal meter doesn't work so I can't set it.

 

[Moleculo]

Use the external meter on the radio that is putting out more than 2 watts and tell us what you see. Some (most) external SWR meters won't calibrate properly when you get down to about 2 watts output, so it's possible you're not getting a good reading while using the cobra

 

[FubarPerson]

My other radio says 1:1 on 1, 20 and 40 with a 6 watt output.

 

[Highlander_821]

Sounds to me like you need to try a different SWR meter. First of all, a 1/2 wave dipole, even if you have it cut for the best resonance, is not going to have a 1 to 1 SWR. Not without some impedence matching device of some sort. Second, There is no way that, even if you had a 1 to 1 at channel 20, you would also be 1 to 1 at the band edges. Just not going to happen.

Are you sure you know how to use the SWR meter correctly? Because your results are not credible.

 

[FubarPerson]

Using a Midland 23-134 meter good for reading swr up to 225 mhz and reading watts up to 148mhz.

I use this meter all the time on my Icom IC-2100 and Yaesu FT-2400 2 meter radio with an Arrow OSJ Model J146/440 j-pole and it reads as it should
(144.00=1.5:1 / 146.00=1.2:1 / 148.00=1.5:1)

I know a 1:1 swr is pretty much impossible, it's the non consistant readings that throw me off. Looks like the cb should stay in the truck where it belongs.