CB Antenna question

[Project25_MASTR]

Hello. Sorry to revive an older post but I just w wanted to thank everyone for their help.

I have solved the issue, turned out I had a bad Antenna. The copper winding that was supposed to be soldered to the lug on the bottom was broken. Hence the off the chart swr I was getting. Once I changed Antenna to the other I get a reading of 1.5 on CH 1 and 1.2 on CH 40.

Can anyone explain the ratio to me? Eg 1.5:1

It's a ratio derived from the forward and reflected power's voltage squares.

SWR = (1+sqrt(Pr/Pf))/(1-sqrt(Pr/Pf))

Essentially it represents a percentage of the amount of transmitted power that is being reflected back at the radio. 1.3:1 = 2.5%, 1.57:1 = 5%, 2:1 = 12.5%, 3:1 = 25%, 5.8:1 = 50%

 

[K7MEM]

Can anyone explain the ratio to me? Eg 1.5:1

It's a ratio derived from the forward and reflected power's voltage squares.

SWR = (1+sqrt(Pr/Pf))/(1-sqrt(Pr/Pf))

Essentially it represents a percentage of the amount of transmitted power that is being reflected back at the radio. 1.3:1 = 2.5%, 1.57:1 = 5%, 2:1 = 12.5%, 3:1 = 25%, 5.8:1 = 50%

To expand on that a little bit, the transmitted power reflected back to the radio is not lost power.

All of your power, minus the losses in the coax, is radiated by the antenna. Initially, with a SWR greater than 1:1, a percentage of your power is reflected back down the coax, When it reaches the radio, that power is reflected back to the antenna, and a percentage of the reflected power is reflected back from the antenna. So, except for the resistive losses on the initial signal, and the subsequent reflections, everything else is transmitted.

An example might be with a transmitter with 5 watts output that is feeding an antenna with 50 feet of RG-8X coax. Even with a perfect match, the coax would present a initial loss of 0.776 dB (-0.818 Watts). However, if the match is not perfect, there will be some loss due to SWR. When the SWR is low (< 2:1), loss due to SWR is usually small and can often be ignored. If you again assume the the above scenario, a SWR of 1.5:1 would cause another 0.054 dB (-0.052 Watts) of loss, for an overall loss of 0.83 dB (-0.87 Watts). That would give you 4.13 W, at the antenna.

In the end, most of your loss is due to the coax, so you should always use the best coax possible. And, the coax should be as short as possible.

Martin - K7MEM

 

[mike_gain]

I was wondering about that tape on the antenna. I guess that wire on the mount is suppose to be the counterpoise.

 

[homerjay]

I was wondering about that tape on the antenna. I guess that wire on the mount is suppose to be the counterpoise.

The tape was to cover the solder joint back up. That was a broken fibreglass whip off my Mack at work, I re-attached the brass nut and re-soldered the cable to it. I use it as a scanner anntenna on my truck. Works very well on the VHF and lower section.

I have since resolved my issue with the SWR on my radio. I mfg a stake hole mount at work and wired it up through the box side and got my swr down to 1.4 on ch 1 & 40 (obviously I dont use the mini antenna).

 

[JayMojave]

Yo homerjay:

How far is the antenna from the cab of the truck now. Fellow distinguished (well semi) radio enthusiasts want to know. Good going. Its always good to hear guys fixing there radio stuff.

many many years ago in a land far far away, um at a local BBQ / potluck / get together one of the locals had his antenna mounted on a tool box just behind the cab, again causing a high SWR. This caused a glitch in the party mod force of many, so something had to be done. First we re-uped our drinks to more octane and second we drilled holes in the top of the trucks roof, for the antenna to be mounted. Third we installed new RG213 Coax the old Radio Shack RG58C had to go. I soldered on the new PL-259 connectors, and checked it out.

This new antenna installation worked quit well the SWR went down, and the antenna got out a hole lot better, many dB's worth.

Jay in the Great Mojave Desert..... One safety tip here, have a sober person make the antenna mounting holes on top of the cab, so that the hole saw doesn't dance across the top of the cab leaving hole saw teeth makers for a foot or so.....

 

[homerjay]

Yo homerjay:

How far is the antenna from the cab of the truck now. Fellow distinguished (well semi) radio enthusiasts want to know. Good going. Its always good to hear guys fixing there radio stuff.

The exact same it was before just now just 3 inches in from the edge of the box. I'll post pictures when I can.

....... One safety tip here, have a sober person make the antenna mounting holes on top of the cab, so that the hole saw doesn't dance across the top of the cab leaving hole saw teeth makers for a foot or so.....

LOL

 

[KC4RAF]

Jay, THAT has to be the classic post yet!

LMAO reading that.

 

[JayMojave]

Hello All and RAF: Thanks for the flowers, glad it was found humorous. But unfortunately its is all true.

One Thursday to Friday we all seemed to say we will have a BBQ at one of the locals 10-20 and have mass quantities ect. Mainly to bad mouth women, polations, each other, and who ever showed up. It got a little out of hand but that was expected.

The better way of drilling a 3/4 inch hole for the NMO antenna mount is using a "Counter Bore" this is a drill bit that has a 1/4 pilot drill bit then four flat cutting blades that cut a really nice clean hole. Well worth the extra cost of a hole saw. Hole saws are a little wildly if your not careful, even if you are careful! Using the counter bore drill still needs care, just barley cutting the 3/4 hole then removing the bite to avoid pulling out wiring and insulation. Its been done.

Again thanks for the flowers.

Jay in the Great Mojave Desert

 

[Project25_MASTR]

Hello All and RAF: Thanks for the flowers, glad it was found humorous. But unfortunately its is all true.

One Thursday to Friday we all seemed to say we will have a BBQ at one of the locals 10-20 and have mass quantities ect. Mainly to bad mouth women, polations, each other, and who ever showed up. It got a little out of hand but that was expected.

The better way of drilling a 3/4 inch hole for the NMO antenna mount is using a "Counter Bore" this is a drill bit that has a 1/4 pilot drill bit then four flat cutting blades that cut a really nice clean hole. Well worth the extra cost of a hole saw. Hole saws are a little wildly if your not careful, even if you are careful! Using the counter bore drill still needs care, just barley cutting the 3/4 hole then removing the bite to avoid pulling out wiring and insulation. Its been done.

Again thanks for the flowers.

Jay in the Great Mojave Desert

Lot to say about that $35 NMO hole saw…