Author Topic: Need CB radio help (Read 3282 times)

mtnbkr · « **on:** April 23, 2005, 05:43:02 PM »

I was 4wheeling with a friend and found out my CB radio isn't working properly. I can receive fine and my SWR runs from 1.4:1 on Channel 1 all the way down to 1.05:1 on Channel 40. However, he can't hear me at all, even from 10ft away. If he does hear me, I was coming through in a very broken up manner with lots of static.

I'm using an old Realistic (Radio Shack brand) radio with a 4' Wilson whip antenna and quality cable. I have a 90degree adaptor at the radio to clear the firewall (radio's in dash). The radio is secondhand and probably 10 years old or older. My guess is the mike, but I want a second opinion before I replace anything.

Chris

P95Carry · « **Reply #1 on:** April 23, 2005, 07:03:53 PM »

Chris - mic would to me seem first to blame. SWR is no prob and your whip sounds like a good one... not sure - but does that one have base loading coil? No biggie that anyways.

10 feet away - sheesh - his set up should be cookin even with std 4W power fromn you... are you using a burner? Does he register you even as just a carrier on his S meter tho - that should tell something?

Whatever - check the plug on mic - in case one wire has fatigued - it happened to me. The ''static'' type noise can IIRC seem excessive if the mic effectively has an open ground thru a failed wire.

Other option - your Tx power tranny may have gone down - happened to me on an old Midland long ago. Buddy's S meter should tell that one. If you are radiating then it's perhaps back to mic prob.

Just thinkin' aloud. Long time since I used my CB gear.

Sergeant Bob · « **Reply #2 on:** April 24, 2005, 01:27:22 AM »

If you have a CB shop near you (many truckstops have shops in or near them), most of them will run a free diagnostic on your radio .
If it turns out the radio needs repairs, I'd be real hesitant about putting a whole lot of money into a Radio Shack CB.
You can get a decent Cobra or Uniden for a reasonable price.

mtnbkr · « **Reply #3 on:** April 24, 2005, 02:51:50 AM »

Thanks guys. The antenna is not base loaded to my knowledge. It's a fiberglass whip and the coil winds up to the tip of the antenna. Dunno if that makes any difference to your diagnosis or not. We didn't think to look at what his SWR meter was saying while I transmit.

There are a couple of radio shops in the area, but I didn't get back in time to visit one. I'm not fixing this radio (unless it's just the mike), but I do need to get another one of the same size. It has to fit in the truck's original ashtray slot.

Here's the installation:

Antenna mount:

Chris

Ukraine Train · « **Reply #4 on:** April 24, 2005, 08:54:11 AM »

I have a Uniden 510 and it's tiny, it would probably fit in your ash tray. I think you can pick one up for like $40, maybe even less used. Maybe you can try swapping mics with your friend.

mtnbkr · « **Reply #5 on:** April 24, 2005, 10:15:54 AM »

The 510 is one I'm considering. I doubt a mike swap would work. The connector on my radio is different than most. Dang RS and their non-standard equipment.

Chris

K Frame · « **Reply #6 on:** April 24, 2005, 11:39:57 AM »

Radio shaft mikes have never worked worth a damn for me. When I had a CB and was using it regularly it seemed like I was replacing the mike every year.

mtnbkr · « **Reply #7 on:** April 24, 2005, 12:00:09 PM »

Unless the local radio guy has a replacement mike or he tracks the problem to another source, I'm replacing the entire unit. I can get the Uniden 510 radio for $50 or less online. Midland makes a similar model for roughly the same price. Any reason to choose one over the other?

Chris

K Frame · « **Reply #8 on:** April 24, 2005, 12:28:54 PM »

Oh, one of the reasons your friend might not be hearing you?

Choice.

Actually, are you sure that you don't have a short in the antenna wire some place?

Have you run the wiring to check for rubbed spots?

Sergeant Bob · « **Reply #9 on:** April 24, 2005, 12:43:18 PM »

For a mobile radio I would definitely go with the Uniden, or a Cobra 19 DX ($54 direct from Cobra, probably cheaper in a truckstop).

mtnbkr · « **Reply #10 on:** April 24, 2005, 04:15:24 PM »

Quote

Actually, are you sure that you don't have a short in the antenna wire some place?

I haven't eyeballed the entire cable, but it doesn't run through any tight spots or anything that would rub it. Also, I wouldn't expect the SWR to tune properly if the antenna was shorted. That would change the effective length of the antenna, forcing me to retune for acceptable SWR. As it was, the system required no adjustments when I checked it the other night (last time I checked the system, it was newly installed).

Sergeant Bob, the Cobra looks interesting. I'll have to track one down. I like the idea of a PA system. I can yell at the old folks when they don't get moving quick enough.

Chris

enfield · « **Reply #11 on:** April 25, 2005, 04:12:43 AM »

One other thing to look at is your coax from the radio to the antenna. Bad (lossy) coax can give you a great SWR reading but still not transmit the signal from the radio to the antenna -- all your signal goes into heating the coax.

I strongly suspect your mic or mic wiring/connection.

mtnbkr · « **Reply #12 on:** April 25, 2005, 05:16:57 AM »

I guess it could be the coax, but I didn't buy cheap coax and I didn't coil it up when I routed it through the engine compartment. How would I check the coax absence of any physical damage?

Chris

Werewolf · « **Reply #13 on:** April 25, 2005, 05:29:10 AM »

A VSWR reading a bit over 1 indicates that you are transmitting RF into the antenna correctly. A high SWR would indicate coax problems, an open in the antenna somewhere along its length or any of a number of other problems. A 0 VSWR means no RF energy leaving the radio.

That said: Antenna mounted on front bumper (looks like the front bumper anyway) ... Wow! If you can verify that the mic is working - there's usually a meter on the radio that will indicate that your voice is actually being picked up (though it doesn't tell you whether or not your voice is actually modulating the RF signal) then there is one other potential problem.

Antenna placement. The antenna is essentially a half wave monopole and mounting it to the car makes it a full wave. Since you have a VSWR of a bit over 1 indications are that the antenna is grounded properly which leaves mount location. Funny locations can result in you getting weird shaped RF propagation patterns. When an antenna is mounted on a bumper you get propagation patterns that look like an egg coming out of the front and rear of the car. Signal strength to the sides of the vehicle can actually be almost 0 especially very close in. Then there's that metal grill directly behind the antenna - that thing can act like a reflector and further direct the RF energy - especially if the spacing between the grill elements happens to be an even harmonic of the RF wavelength.

Try moving the antenna to the trunk lid or better yet the roof - Assuming you've verified everything else is working properly.

mtnbkr · « **Reply #14 on:** April 25, 2005, 06:05:19 AM »

The position is a compromise since I do not want a magnetic mount, nor do I want to drill holes in the body or roof. Besides, my friend's antenna is mounted on the back of his jeep using a shorter antenna. When he was coming towards me (antenna on the other side of the jeep from me), I could hear him clearly. When he was in front of me (antenna on my side and less than 20' away), I could hear him clearly. When we first established radio contact with each other, he was half a mile away with a large hill between us. At no time could he hear me clearly, if at all.

The only meter built into the radio is an LED that tells me if I'm in transmit or receive mode. I used an external SWR meter to tune the radio.

Chris

enfield · « **Reply #15 on:** April 26, 2005, 07:08:57 AM »

Quote

A VSWR reading a bit over 1 indicates that you are transmitting RF into the antenna correctly. A high SWR would indicate coax problems, an open in the antenna somewhere along its length or any of a number of other problems. A 0 VSWR means no RF energy leaving the radio.

Not quite true.

A VSWR of 1:1 means that the coax is presenting a 50 ohm load to the transmitter, assuming the meter is between the radio output jack and the coax input. It doesn't mean that the coax is delivering the power to the antenna, and it doesn't mean that the 50 ohms is purely resistive.

A high (3:1 or higher) VSWR indicates a mismatch between the input to the meter and the output of the meter. It doesn't tell you where the problem is.

A VSWR of zero is impossible. VSWR goes from a minimum of 1:1 to a maximum of infinity:1.

You can check the quality of the coax by using a $350 antenna analyzer, or by using a complicated manual procedure, or by simply changing out the coax and seeing if it makes a difference. If I didn't own an analyzer, I'd swap out the coax. And whether changing the coax cured the problem or not, I'd leave the new coax connected.

mtnbkr · « **Reply #16 on:** April 26, 2005, 09:30:42 AM »

Well, I have a new radio now. I bought a Uniden PRO510XL and installed it. I can't get SWR quite as low as I did with my Radio Shack special, but it's low enough (high of 1.5:1 at Ch1 down to 1.3:1 at Ch40). Mounting was a female dog as the new radio's mount holes are a bit off from the old radio's. To make it work, I had to fabricate new mounts. It's not as clean as before. It's a tad crooked, but in a way that makes it easier to see from the driver's seat.

Thanks for all the help guys.

Chris

cfabe · « **Reply #17 on:** April 26, 2005, 09:45:32 AM »

Does the new radio work, though. That's the real question...

mtnbkr · « **Reply #18 on:** April 26, 2005, 11:00:22 AM »

Well, it receives rather well. I haven't tested transmission yet since I'm too far from the nearest interstate for people to hear me. I'll try it tomorrow on the way into work.

Chances are, it'll work just fine though. I heard from my father in law (Radio Shack CB's prev owner) that it gave him trouble as well.

Chris

Werewolf · « **Reply #19 on:** April 26, 2005, 11:48:20 AM »

Quote

A VSWR of 1:1 means that the coax is presenting a 50 ohm load to the transmitter, assuming the meter is between the radio output jack and the coax input. It doesn't mean that the coax is delivering the power to the antenna, and it doesn't mean that the 50 ohms is purely resistive.

A high (3:1 or higher) VSWR indicates a mismatch between the input to the meter and the output of the meter. It doesn't tell you where the problem is.

A VSWR of zero is impossible. VSWR goes from a minimum of 1:1 to a maximum of infinity:1.

SWR stands for standing wave ratio. It is the ratio of the power of RF going to the antenna vs that reflected back back into the RF amp due to impedance mismatch. Antenna impedance is a function of the total impedance (Resistive, Inductive and Capacitive) of the antenna which is generally a function of antenna length and transmission line characteristics. High VSWR's generally mean one of two things high resistance between the transmiter and the antenna (bad coax) or an antenna whose physical/electrical length is not an even harmonic of the RF wavelength being transmitted.

Cheap CB VSWR meters may not be able to go less than 1 but the tuners we used in the Navy to tune our HF xmitters would. The only time they'd go less than 1 was to indicate a fault - usually a coax connector was shorted out due to seawater corrosion.

I suspect CB antennas do represent a 50 ohm impedance because CB radios generally use RG52 coax. I've worked on a variety of both low power and high power xmitters (like 500,000 Watt HF xmitters at Pensacola NAS and a 50,000 watt UHF TV xmitter at KOKH in OKC) that had impedances from 50 all the way up to 1000's of ohms. One radar I worked on had a xmission impedance well over a megohm.

The point of my original post was that if the VSWR meter was in the bit over 1 range then RF energy was reaching the Antenna and that the antenna was tuned close enough for government work.

News:

Author Topic: Need CB radio help (Read 3282 times)