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Two questions,

KB1TPX

KB1TPX

W1MBZ
Jul 22, 2008
493
10
48
71
Warwick, RI 02818
1) When runnning a bi-linear amp, is it better to calibrate the swr with the amp on or off?
2) Is a power mic necessary when the radio has a mic gain, as with the Cobra 139 with "Dynamike". My power mic screeches no matter what the setting on the rig or the mic.
 
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"Bi-linear" has nothing to do with it. As long as the meter is after the amplifier you should be okay. Just remember that you have to REcalibrate the meter whenever you change power level, or when you change frequency more than a few channels in either direction.
 
Yeah, thanks, I know about calibrating for each frequency, I was just wondering wether or not it made any difference with the amp on or off, as the needle reacts differently with the amp on then with the amp off. Now I know.
Thanks again.
How about the mic gain question, any thoughts?

Mike
 
You should check your SWR's with the amplifier in line and the SWR meter between the amp and the antenna. You'll want to check with the amplifier off and then turn it on, recalibrate for the extra output and then check the SWR's with the amp ON.

Often with the extra power cheap coax or a cheap antenna can't handle the power and that's where you get high SWR's.

And a power mike is never "necessary" it's something people add if the stock mike isn't picking up their voice really well. Mic gain control doesn't actually increase the gain, it's an adjustment to reduce the gain of the stock mike when you don't want to pick up as much road noise or similar.
 
As a general rule, SWR does not change with power levels. However, in a mobile environment there are other variables which come into play that can cause havoc. Reflection from nearby big trucks for example, is something that can cause big problems at the right frequencies or power levels. In a home environment, I seriously doubt you will see any change in SWR with the amp on. If you have an issue with an antenna or coax that can't handle the power, it's likely that you won't notice any problem on the SWR meter until it's too late and you've already turned your antenna into a giant fuse.
 
A typical SWR meter will show an SWR change when different output power levels are used. Not because the SWR is actually changing, usually, but because the typical SWR meter isn't capable of a wide range of input power without some compensation (re-calibrating/changing the slug/etc.). The most common instance where a change in power level would make a difference in the actual SWR meter between the amplifier and the antenna is when some portion of that 'equipment string' changes impedances. (Output of the amplifier not really 50 ohms as it should be, probably the most common, for whatever reason.) If any SWR meter (adjusted correctly) sees 50 ohms coming in one side of it, and sees 50 ohms on the other side of it going out, the SWR is 1:1. Changing the power level without correcting for it, means the meter is just being over loaded, so is operating out of it's "normal" range. Then there's really no telling what you might see. (Why they usually have different power range switches, right?) If there's a change in SWR it just means there's been a change in the impedances from one side of that meter and the other side. Find-n-fix the change where the change occurred... or make sure you've set up the meter correctly.
- 'Doc


been a long 'day-off' but had to work anyway. Top that off with trying to fix a Vista and program problem. At this point nothing is making sense so to @#$$ with it, I'm going to bed. Best advice is to speak firmly with your meter. Tell it what you enjoy doing with large hammers and naughty meters... Doesn't help, but it's satisfying as @#$$ !
 
Wow, a lot of excellent advice!
It is definately a cheap meter, which is situated between the amp and the antenna.
I have RG 8U 13-AWG type CL-2 going from the rig to the amp, then 52ohm from the amp to the meter an finally RG58/U going out to the antenna.
Should I use all RG58 from radio to amp to meter to antenna?

Mike
 
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Hey Big Kahuna, I had a friend from Hawaii nick named Pineapple, who I was in the Seabees with. His CB handle was Big Kahuna. He lived in Rhode Island last I knew.
Where are you located?

Mike
 
Personally, I wouldn't use anything besides RG-213 for anything except maybe the jumper from the radio to the amplifier. For that, the RG-58 should be fine, IF it is made by a reputable company (Belden, e.g.).
 
I wouldn't even use it BEFORE the amplifier.

Now some Tandy/RS coax is often pretty good, like their RG-8X. Since they don't make the coax themselves (they contract), there are different manufacturers, and some of them might just take a shortcut now and then.
 
Well I do intend to get all identical jumpers and lead-in coax to make it uniform from start to finish. But for now, it works well. I was told that I was "blowin' smoke" down in Tenn., not bad considering I'm on the seacoast of NH.
 
7732granitestate said:
I was told that I was "blowin' smoke" down in Tenn., not bad considering I'm on the seacoast of NH.
Click to expand...

Thats funny because usually if you see a puff of smoke coming from you amp, it stops working.

Anyways, one reason you see higher VSWR with the amp on is that the amp can be putting out harmonics if you drive it too hard. Antennas reflect harmonics, so they show as added VSWR. This applies to pill amps. Tubes tend to have less harmonics.
 

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