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Measure swr with amp and filter

L

Littlears

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Aug 16, 2020
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Hi all. New guy here. Excited to be getting back into CB / GMRS (maybe Santa will bring me one).

I’m trying to figure right the right spot to measure SWR

my setup is:

- Firestick 4’ Mounted to my wrangler JKU tailgate. Ground straps to chassis.
- cobra 75 radio, grounded to chassis.
- RM Italy KL35 amp grounded to chassis.
- RM Italy 27/586 low pass filter.

connections / signal flow are :

cobra 75 —> rm Italy amp —> low pass filter —> firestick antenna.

Before I added the amp and filter, I was measuring 1.4-1.6 SWR between radio and antenna.

Now with the amp and filter inline, I’m confused where to measure swr. I’ve done some reading here about harmonics and honestly kinda lost.

any help you kind folks can provide is appreciated. Thanks!
 

You should measuse SWR at the antenna feedpoint.
1. Use measurement device which can subtract coax from measurement.
2. Use 180 degrees coax line.
Mike
 
Welcome! Our set up is similar. I am glad you got that filter. I am running the 203 and without that filter the harmonics would make SWR rise.

So here is how I check for SWR...Radio, Amp, Filter, SWR meter, antenna. So place the meter between the filter and antenna.
 
Littlears said:
awesome. Thanks for clarifying !!
Click to expand...

Ya ma, no worries. Give us an update and pics. Some of us are pic whores!


Littlears said:
Oh ya. Been reading this site a bunch and have gotten myself some braided tinned copper & connectors to make bonding straps - whenever I can find the time:)
Click to expand...

Awesome! Time is always the killjoy. Don’t forget the star washers so they “bite” into the metal.
 
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ForestRunner98 said:
Ya ma, no worries. Give us an update and pics. Some of us are pic whores!




Awesome! Time is always the killjoy. Don’t forget the star washers so they “bite” into the metal.
Click to expand...
I was wondering about that. I have normal tinned copper connectors (look like a doughnut with the center punched out so a bolt can pass).

If I’m just bonding under car parts presumably I don’t need to worry about paint or similar from stopping the bonding from working, right?

ps - do I need to worry about different metals and the advancement of rust?
 
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You can put the star washer against the paint. Also test and check if there is continuity at both points.

It shouldn’t rust. The star washer should bite into the zinc under the paint. I put some dielectric grease where metal was exposed. Not on the exhaust though. If you do bond the exhaust just remove any surface rust on the tubing.

Be careful!! I initially added a radio for off-roading but soon got bit by the radio bug!!!
 
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The AM dead key on that Cobra 75 will be too much for that little KL-35. The Cobra 75 is a marginal radio at best, it was designed with convenience in mind. To be honest that KL-35 is just about worthless. If you must run that combo set the SWR with the radio only in line. Then add the amplifier and run it because it is what it is at that point. You can skip that in-line filter as well for that set-up.

About 18 years ago I tested a Cobra 75 into a Silver Streak 250 amplifier and the dead key was even too high for that amp. A co worker ran it for about a year and then moved on to something else.
 
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9C1Driver said:
The AM dead key on that Cobra 75 will be too much for that little KL-35. The Cobra 75 is a marginal radio at best, it was designed with convenience in mind. To be honest that KL-35 is just about worthless. If you must run that combo set the SWR with the radio only in line. Then add the amplifier and run it because it is what it is at that point. You can skip that in-line filter as well for that set-up.

About 18 years ago I tested a Cobra 75 into a Silver Streak 250 amplifier and the dead key was even too high for that amp. A co worker ran it for about a year and then moved on to something else.
Click to expand...
9C1Driver said:
The AM dead key on that Cobra 75 will be too much for that little KL-35. The Cobra 75 is a marginal radio at best, it was designed with convenience in mind. To be honest that KL-35 is just about worthless. If you must run that combo set the SWR with the radio only in line. Then add the amplifier and run it because it is what it is at that point. You can skip that in-line filter as well for that set-up.

About 18 years ago I tested a Cobra 75 into a Silver Streak 250 amplifier and the dead key was even too high for that amp. A co worker ran it for about a year and then moved on to something else.
Click to expand...

prior to getting the KL I tested my dead key wattage (radio —> meter —> antenna ...with a 1.5-1.6 swr @1&40) was getting about 3w (maybe 3.5 max) dead key with engine on and 2 watts max engine off (voltage supply differences with engine on / off). The KL spec says it’ll do up to 5 watt input ?

why skip the filter ?

I agree , the cobra doesn’t have all the features as most of the other radios, but the wrangler interior is tight!
 
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Here are the specs for the RM KL-35. It is a very small amplifier that will run much better with a 1-1.5 watt dead key input. To make a difference on your signal you must DOUBLE your power to gain HALF of an S unit. You would be better off with a 150-250 watt amp. The KL-35 will make little to no difference in your signal. As far as the filter goes, if you were running say 200-1000 watts and you were concerned your signal was getting into your neighbors tv or stereo then yes, run a filter. In your case a filter is completely unnecessary. Your in a mobile and your only doing 30 watts.
  • Freq. Range: 25-30 MHz,
  • Supply:12-14Vcc,
  • Input energy: 4A,
  • Input Power: 1-5w,
  • Output Power:25-35 watts,
  • Input SWR: 1.1-1.5,
  • Mode: AM/FM,
  • Fuse: 5A
  • Dimensions: 109x81x35 mm
 
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Many people who are tight on space and see the Cobra 75 and think great I need this. In reality the Cobra 75 is a dismal performer. With a small antenna you will be lucky to get 1-2 miles, an FRS radio can do that. A good use for the Cobra 75 would be toss it in the car for a vacation to monitor channel 19 and get a bear report from time to time.

There are plenty of smaller size radio's that are much better and even cost less. A good step up also would be a small AM/SSB radio. Good luck.
 
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9C1Driver said:
Many people who are tight on space and see the Cobra 75 and think great I need this. In reality the Cobra 75 is a dismal performer. With a small antenna you will be lucky to get 1-2 miles, an FRS radio can do that. A good use for the Cobra 75 would be toss it in the car for a vacation to monitor channel 19 and get a bear report from time to time.

There are plenty of smaller size radio's that are much better and even cost less. A good step up also would be a small AM/SSB radio. Good luck.
Click to expand...

PRESIDENT is a high quality brand with several quite small radios. I think there’s even a a Wrangler Mount bracket for that size. (Tera?).

The Johnny III has some nice features (VOX & two types Squelch). BELLS CB carries it.

The real problem is a decent antenna mount for the Wrangler. (For correct performance).

.
 
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