tuning cb radio

[cbrown716]

I just got my astatic pdc1 swr/power meter and need some trouble shooting. I'm running a midland 5001z with a 4' k40 4' top load and there's an 18ft coax. I used a mirror mount that's screwed in to the bed mount in my truck and the antenna is not grounded.

I had a high ratio on my first go and after some screwing around I removed the spring and got a reading of 1.4 on channel 1 and 1.9 on channel 40 but I can't move it down any further without lowering the mount. I'm wondering if someone knows how I can resolve this. I'm fairly new to cb radios so I'm not 100% sure on what I'm doing everything I've learned I've had to Google. Thanks in advance!

 

[SCPD]

Whip it good

I just got my astatic pdc1 swr/power meter and need some trouble shooting. I'm running a midland 5001z with a 4' k40 4' top load and there's an 18ft coax. I used a mirror mount that's screwed in to the bed mount in my truck and the antenna is not grounded.

I had a high ratio on my first go and after some screwing around I removed the spring and got a reading of 1.4 on channel 1 and 1.9 on channel 40 but I can't move it down any further without lowering the mount. I'm wondering if someone knows how I can resolve this. I'm fairly new to cb radios so I'm not 100% sure on what I'm doing everything I've learned I've had to Google. Thanks in advance!

From what I'm reading this, you need to shorten the length of the whip on the K40.
First off, you got the whip all the way into the base, yes?
If not, go do that. Loosen it up, shove it down in there as far as it will go, and check the match.
Did it get better?

As far as what you got now, 1.9 to 1 is good enough for me. Your call.
If you're really striving for a 1.1 to 1, then cut about a half inch off the bottom of the whip, and check your match.
Keep cutting a half inch off the bottom of the whip until you get 1.1 to 1, or whatever makes you happy.
You're not gonna get 1.1 to 1 across the band. Trim the antenna for the best match on 19, and be good to go.

 

[cbrown716]

Thanks for the reply. I might be stuck with where it's at now I realized the spring is useless (lowest I got with the antenna was 2 and 2.5) on it and I can't tune it any lower and chopping it down would be too much work for that little extra bit. Does not having the antenna grounded make a difference?

 

[AC9KH]

I used a mirror mount that's screwed in to the bed mount in my truck and the antenna is not grounded.

That's your problem if you're using an antenna that's designed for a ground plane. Ungrounded antennas are never going to be resonant and if you can get the VSWR to 1.5-2.0 you're doing about as good as you can do.

The ideal place for your antenna on a truck is in the middle of the cab roof if you want a 1.2 VSWR. And make sure when you check and set it that you're out in the open where you won't get any reflected RF from buildings, etc.. You can set the VSWR to 1.2 with the antenna in the middle of the cab roof, then move the antenna to the back of the cab where part of the vertical is shielded by the rear of the cab with just the tip sticking above, and it will raise your VSWR to 1.8. There's more to it than just tuning the length of the vertical. An antenna designed for a ground plane NEEDS that ground plane to work properly.

An example: I have a Wilson 1000-series with a 62.5" whip on my extended cab pickup. It came with a magnetic mount and it worked so-so with my sideband radios, but not what I expected. I could barely get 20 miles base to mobile on sideband. I ordered the surface mount for it and drilled a 3/4" hole in the cab roof so it has a decent ground plane, and scrapped the magnetic mount. Whole different unit - instantly almost twice the range with S-7 at 20 miles base to mobile.

 

[jonwienke]

Does not having the antenna grounded make a difference?

Only everything. It's similar the the difference between touching an electric fence grounded or not grounded.

 

[cbrown716]

Awesome thanks for the help. What's the best way to ground it?

 

[AC9KH]

Only everything. It's similar the the difference between touching an electric fence grounded or not grounded.

That's a good example. The electric fence is a capacitive discharge, while an antenna is an AC electrical circuit like your 120V wiring in your house. The circuit must be complete in order for the antenna to work. Just like in your house wiring where you have a "hot" and a neutral that provides a return path to the generator, the antenna has a "hot" (center coax conductor) and a "neutral" (coax shield) for a return path to the transmitter (generator). And just like your house wiring where if you run the hot to a light bulb with no neutral, the light bulb is not going to light up.

So-called "no ground plane" antennas really use capacitive coupling on the ground, whether it be a magnetic mount or some other type. But regardless, it has to be there and if you have a high impedance on your return path connection to the transmitter you are going to have a mismatch between your antenna impedance and feedline. That impedance mismatch is what SWR is.

 

[AC9KH]

Awesome thanks for the help. What's the best way to ground it?

I assume your mirror mount is one of those "donuts" that you can split in half - and the coax center conductor goes thru one hole and feeds the antenna vertical with a compression washer, while the coax shield goes thru an opposite hole and is supposed to be grounded to the vehicle steel?

You have to make sure, whatever type of coax termination is used at the antenna base, that the coax shield is connected to the steel mass in the vehicle. That way the current produced by RF radiation will have a better (lower impedance) return path to the transmitter via the coax shield.

I should note that your vehicle steel mass is not really the ground plane for a monopole antenna like the K40. The vehicle acts as a sort of capacitor between the antenna radiator and real ground plane, which is the earth. But the RF electrical circuit in the vehicle must be complete in order for it work right.

 

[cbrown716]

That's the exact mirror mount I have. I believe it was manufactured by truckspec. So the way to properly ground the antenna would be through the shield on the coax? I don't plan on keeping the k40 for a whole lot longer eventually I'm looking at a dual mount 3' firestik.

 

[cbrown716]

I'm also wondering, my cb has a ground that it uses since it won't turn on without the ground connected would the coax ground the antenna back to that or do I have to cut through to the coax shield at the antenna and run a ground to the chassis?