Penetrater HB-150 and high SWR (CB)

[scatrbrain]

I have a Penetrater HB-150 that I have put in line. With an SWR meter in line between the amp and antenna with amp OFF I get a very low (almost flat) SWR reading. But when I switch the amp ON the SWR goes off the scale. This is the same car, same antenna etc that I have used for years with a Palomar with NO SWR problems. People have tried to help by telling me that I'm not grounded correctly etc, but I don't buy it since everything is grounded very well already and has worked for years until I put this amp in line.

What I have heard is that many CB amps (the Penetrater HB-150 has been singled out for this by the way) have not been built correctly with low pass filters etc and put out a secondary harmonic which has a frequency way above what the antenna can transmit, in effect reflecting that secondary harmonic back down the antenna and giving you a high SWR reading. I have heard these readings referred to as "false SWR readings" but I'm not sure that means its not going to eventually burn up the amp.

My question is, does anyone here have experience with this? Can you tell me if this second harmonic reasoning is plausible? Is there nothing I can do about it? Could I be WAY off, and the real reason be a ground problem even though I haven't had any ground issues in the past with a different amp?

Oh, should I mention that I have tried different coax, and two different antennas including my A99 which also has great grounding and a flat SWR reading.

Thanks

 

[nomadradio]

Your SWR meter is being fooled.

The antenna's SWR is not changing.

That amplifier, as you speculate, contains NO filtering.

Add to that the primitive design, and its advanced age. Odds are that it is pumping out extra harmonic frequencies in addition to RF on the channel you THINK you're transmitting on.

The radio's output is presumably fairly clean. It DOES contain some amount of harmonic filtering. Very, very little of the radio's wattage will be at those "extra" frequencies.

The deal is, that at 54 MHz or 81 MHz or 108 MHz, your antenna has a very high SWR. No need to put a 81 MHz transmitter on it, just take my word for it.

The radio alone shows a low reading because nearly all the wattage it produces is ON the frequencies the antenna is built and tuned to accept. If you pump those extra frequencies into it, the SWR at THOSE ffequencies WILL be high.

And that's what fools the SWR meter. It can't tell the difference between RF on ONLY one frequency, and RF power that is spread across four or five of them.

Since the SWR is so high at those harmonic frequencies, it only lakes a little bit of power on those frequencies to give you a LARGE reflected power reading. The antenna is rejecting the WHOLE portion of the amplifier's wattage that is NOT 27 MHz. Drives up the reflected reading.

One possible cause would be for the two RF power transistors to be out of balance. If one of them is pumping more power than the other, this WILL create harmonic frequencies. Odds are that it's built with RF power transistors that mount on one threaded stud. Adapting that kind of circuit board to the newer flange-mount parts is proably not worth what it would cost.

One thing I do remember about those is that they take only a tiny amount of drive power. Radios were wimpier, on average 30 years ago, and the "ceiling" for the radio's peak power may well be under 15 Watts. Any peak power over that will usually be clipped off of the radio's AM waveform. Kinda like slicing off your audio peaks at the neck (or waist). Makes 'em sound "tight" unless you flip the high/low switch to LOW.

The lower wattmeter reading disappoints the operator, and eventually the heat from running excess drive on "high" damages one (or both) of the 30 year-old transistors.

The real fun comes when you spend big bucks for new RF power transistors and the 30 year-old relay goes bad a week later.

73

 

[scatrbrain]

Thank you!

nomadradio,

Thank you for the reply and the wealth of info you gave me. I had done a lot of research on this and I suspected that this was the problem, your reply gives me some validation.

Can I assume this is going to eventually going to burn the amp up? Or does the reflection of the secondary harmonic not hurt the amp? I assume its pretty old, and I dont know its history (weather or not its been worked on, pills repaced etc). **I re-read your reply and I think you answered this question already**

I forgot to mention that I did in fact have the radio to be used with this amp (Uniden PC122XL), tuned down to key about 2 - 5 watts because I thought thats all the amp could handle on the imput side.


Thanks again for your reply!