The AM Forum

Hi !

One of our community (K1KBW - Bob) picked up a Palomar 500 (1000 watt) SWR / Power meter.  It appears to work fine at 10 meters, but on 75 meters the sensitivity (and therefore the accuracy) is extremely low.

Anyone have any experience with this and/or suggestions (other than get a real meter!!!) as how to make this work?

Bob has a Bird that he can use to calibrate this meter.

Thanks and Regards,

Steve

http://www.cbtricks.com/radios/palomar/model_500/graphics/palomar_model_500_om.pdf

Steve
Bob

Assuming the schematic that was pointed to by Bill (KB3DKS) is correct...

Note that there are actually two separate couplers in this design. One is a directional coupler (essentially the same design concept as a Bird slug), and the other is a non-directional coupler (for the higher power measurements).

Step 1: Check the non-directional coupler

a. Set the power-SWR switch to the the "power" position
b. Set the range switch to either 0-100W or 0-500W (or whatever the high range is)
c. Inject an appropriate amount of power
d. If the power reading is very frequency selective, check the components consisting of the 1N99 diode (I.e. is the side that is supposed to be connected to ground actually connected to ground?), the 0.001uF capacitor, the potentiometer (whichever one you are using), the grounding of the left side of the meter, etc. Keep an eye out for a loose connection that is being bypassed by just a small amount of capacitance.

Step 2: If the power measurements are okay (not very dependent upon the operating frequency), then reduce the power to somewhere between 0-10 Watts, and change the power selector switch to the 0-10W position. This will test the other coupler (the directional coupler). Again, look for a loose ground. Also, make sure that the 200 Ohm balancing pot is not set too close to 0 Ohms.

Stu

That's the hook, Stu. Palomar doesn't spec the frequency coverage in the manual. This meter was intended for CB use. That's why 10 Meters works FB. The stripline coupler is going to be near dead at lower amateur frequencies. There may not be enough coupled RF to get the diodes to conduct along the scale shape printed.

It might be necessary to re-work the coupler and use a toroid transformer around the main line.

21 to 30Mhz should work OK FB!

Find an old ARRL SSB handbook showing how to duplicate the Collins wattmeter. I think it uses a T68-2 core. I built one years ago with dual meters that is in line all the time. I left the antenna connected during a storm once and took out the reverse power diode. Pretty accurate against a bird 160 through 10m. I calibrated the meters for 2KW full scale with no circuit changes. More power would require more turns on the core to step down current. Circuit layout is critical. I used a small minibox.

He might be able to cut a square in the PC board right in the middle og the primary line and drop the pickup coil in it then bridge thru the coil with a piece of insulated wire.

There's a whole bunch of ways one can construct it.

But I'm quite sure the stripline, as is, was only intended for CB!

Bud (WD8BIL):

I agree that the coupler design might be, intrinsically, more frequency selective than it has to be.

In the schematic, it appears that the shield (or maybe it is just a parallel trace on a circuit board) of the coupler is grounded at both ends.

I don't know what the coupler looks like physically, but the sensitivity would likely go up if one end of the shield/trace were taken off ground. I assume that there is a path through the case, much further from the pickup lines, that would serve the purpose of completing the RF circuit... particularly at lower frequencies. If not, one can always add a wire, connecting the input ground to the output ground, that is much further away from the center conductor and the pickup wires.

Frank and Bud

I also agree that Bob could fabricate a new coupler using ferrite cores (I use such a homebrew directional coupler in the shack; and it works FB feeding an oscilloscope)... but the issue is whether the meter scale will match the characteristic of the new directional coupler + diode-based peak detector. It might. If the output of the directional coupler is several volts (peak), and if the meter scale is reasonably close to square-law (1.414 x the current through the meter => twice the reading on the meter scale), then it will be reasonably accurate. Anyway, perhaps that is a lot of trouble to go through... just to get his unit to work.

Stu

IIRC correctly, the coupler is all stripline on a G10 material board and the SO239s solder directly onto the (thru) the board to the primary.
Palomar used 2 different circuits. One is the one posted and the other (older) uses 2 secondary traces. One for forward and one for reflected power.
On the schematic posted here there are 2 strips. The short one is for the 2 higher power scales and the long one with the balance pot is used for the 0 - 10W scale and reflected power only.

 Older Circuit  (http://bama.edebris.com/manuals/palomar/500)

STEVE: Do you have the meter or is it still at Bob's. Pictures would be helpful to know what we're dealing with and how best to go on.

The coupler I see in this circuit is a Monimatch. This circuit, adapted from Lew McCoy's original design, is VERY frequency sensitive, but if used only at CB freqs it is stable enough to  cover that band. However, on the lower freqs, such as 80 and 40, the coupling in that circuit is so low as to be practically useless.

The ideal circuit for the same purpose is Bruene's design, using two ferrite current transformers, and connected such that one responds to the forward wave, and the other the reflected wave. The ones I built many years ago appear to be totally frequency INsensitive from 160 through 10m.

Bruene's article describing his circuit appears in QST, "An Inside Picture of Directional Wattmeters,' April 1959,  p24.

Hope this helps,

Walt

Some of the commericial Collins directional couplers also had variable equalization capacitors to reduce the increased sensitivity at higher frequencies.

The directional couplers are common items at hamfests; they were used in a bunch of different Collins products.  I have a few that appear to have come from the KWT-6.

In any event, life's too short to mess with junk like that  ;D

I used glass piston trimmers to balance my circuit. Layout is critical to get a good balance. I calibrated my unit for 200 watts reflected and 2000W forward.
The Collins circuit only uses 1 current transformer and seems fine over the hF range. I used 1N5711 diodes. I would think the dual transformer directional coupler would be flat over a wider frequency range. There was also a QEX article a few years ago on these couplers.

I remember those Palamar wattmeters from the late 1970's CB days. Actually, they have nice looking meter movements and are attractive looking units.

I would wind up a simple directional coupler using a wire thru coax and two diodes (forward and reverse) to drive the exisiting op amp circuit -- or do the same thing with a toroid. Both circuits are easily found on the web.

I'll bet the existing circuit board coupler would flash over when the typical 24 pill class E rig is put to it anyway... ;)   YAAAYYYYY - ZOOOOOORCH!

T