I have an old Magnadyne SWR-72 for 11 meter (CB). I want to modify it so I can use it on 2 meter (VHF).

The only frequency sensitive components I can see are a couple .001 caps in parallel with a couple 150 ohm resistors. Can i change the meter's frequency response by changing the RC combination in the circuit?

I would submit a photo but I don't know how to attach to this message. Thanks in advance.

"joseph2010" wrote in message
...

I have an old Magnadyne SWR-72 for 11 meter (CB). I want to modify it
so I can use it on 2 meter (VHF).

The only frequency sensitive components I can see are a couple .001 caps
in parallel with a couple 150 ohm resistors. Can i change the meter's
frequency response by changing the RC combination in the circuit?

I would submit a photo but I don't know how to attach to this message.
Thanks in advance.

That appears to be the same as several types of SWR meters with a differant
name. Most of them were rated to go to 150 mhz. Some were in about the same
box with two meters on the box and no forward/reverse switch. I have used
one up to 220 mhz and it seemed ok for adjusting for minimum SWR. They will
not work at all at 440 mhz for swr.

The resistor is to match the internal line to the impedance. The .001 cap
is just an RF bypass. The main things that determin the frequency are the
two wires that are parallel to the wire going from the centers of the two
pl259 connectors. If too short, they will not pick up enough signal to set
the meter to full scale. If too long, they will not respond to the
direction of the forward to reverse power.

If the meter is built like that, it will probably work from 0 meters to two
meters just as it is.

Ralph, Thanks for your informative reply. You're right, there were several manufacturers that made these units (Midland, Pace, Numark, etc.). I did try to hook it up, but the SWR was 1.3:1 (almost the same as I got with a dummy load), which I thought was too good to be true, but maybe it was due to the low wattage being used (4 watts). I'll get some proper cables built and go from there. Thanks again.

"joseph2010" wrote in message
...
Ralph, Thanks for your informative reply. You're right, there were
several manufacturers that made these units (Midland, Pace, Numark,
etc.). I did try to hook it up, but the SWR was 1.3:1 (almost the same
as I got with a dummy load), which I thought was too good to be true,
but maybe it was due to the low wattage being used (4 watts). I'll get
some proper cables built and go from there. Thanks again.


If you can get two 50 ohm dummy loads you can put them into a T connector
and the swr should be around 2:1. That is one way to run a quick check.

Some times using low power will cause the swr to show differant as the
diodes will not have enough voltage to act the same as they do with a higher
voltage. That is called the square law reagon. While the readings may not
be accurate, the lower the reverse reading, the better.

I should qualify that statement about the SWR. The actual SWR will be the
same nomater what power you use, but some meters will show a differant
ammount due to the way the diodes act.

Depending on the antenna and the ammount of coax, the swr can be shown as
very low. If you have around 100 feet of rg58, the swr the meter shows will
be very low and not too much you do at the antenna will change it.
I made that mistake about 40 years ago when I built a 2 meter antenna and
fed it with 100 feet of rg-8. The swr was about 3:1 and nothing I did would
change it. Turns out that the center wire was not hooked to the antenna.
The coax has enough loss that it will only show about 3:1 with nothing
connected to it at 2 meters.

I just read the following, and I'm wondering if it's true?

I'm not sure I believe this, what do you think? thanks in advance

"joseph2010" wrote in message
...
I just read the following, and I'm wondering if it's true?

I use a CB SWR meter to check 2M antennas all the time. There is a
trick to it however.If all you want to to is check SWR on your 2M
antenna, you don't necessarily have to buy a dedicated VHF/UHF SWR
meter. What I keep in my toolbag for that is a cheapie $5 hamfest
special CB SWR meter. They really don't work well on 2M but there's a
trick you can do that will net a reasonably accurate SWR reading on 2M
with one of these meters.

What you do is connect up the meter as usual, key the rig with the
switch in the forward power position, set the adjustment for full scale.
Now, without touching anything, swap the coax connections so that the
rig is connected to the "ANT" side of the meter, and the antenna is
connected to the "XCVR" side. The reading you see on the meter will be
very close to your real SWR. The closer to 1:1 your SWR is, the more
accurate it will be. It would be more convenient to have a real SWR
meter or antenna analyzer if you do a lot of testing, but for a quick
antenna check after a mobile install or whatever, the $5 CB meters work
OK.

I'm not sure I believe this, what do you think? thanks in advance

For the meter you have, it is mostly true. I don't know if it is really any
more accurate,but it will work just fine. Usually when you get under about
1.8 or 1.5 to 1 that is close enough. The main thing is just to tune for
the lowest reflected power. As long as it is low enough the rig puts out
full power, that is the main thing in most cases.

Most any of the SWR or directional watt meters will work fine in either
direction. You can take the meter you have and hook it up the way you are
suspose to and see what you have. The reverse the coax. You also have to
keep it in your head the forward/reverse switch will work backwards. That
is you set it for reflected power and adjust for full scale. The switch it
to forward to read the SWR. That is one way to run a quick check to see if
the SWR meter is working like it should. You may see a small differance,but
should not be too much.

The meter you have has two coupling wires. One is used to measuer the
forward power and one measuers the reflected power. They should be
exectally the same. Those 150 ohm resistors should be exectally the same
also, but they are ofter eithe r 5 or 10 percent resistors. As with most
mass produced items, on line may be a fraction of an inch longer or closer
the the center line. This is not usually a problem. After all , this is
not a lab grade instrument. Even the much praised Bird meter is not really
that accurate if you read the specifications.

Watt meters are not normally needed to be super accurate. I have a service
monitor that origionally cost around $ 50,000. Bought it for under $ 1000
as it was mainly for the old cell phone stuff that is not used any more, but
it also works great for other servicing.

The frequecny readout is almost dead on. The specs is something like being
within 85 Hz at 850 Mhz. The wattmeter is still only rated for about 5%.

Thanks again for your educational and informative response. I have certainly learned a lot with your help. Thank You!

As luck would have it, today I picked up another one with 2 UHF cables at a thrift store for $5, I couldn't pass it up (the cables alone were worth it) . This one is a Midland 26-126 and it's damaged. One of the 150 ohm R is burned and open, not sure of any other damage. I'm going to replace both resistors and try to get a matched set (close as possible). I have yet to test the meter movement, that's next.

Thanks again Ralph. It is appreciated.

In article ,
joseph2010 wrote:

Ralph, Thanks for your informative reply. You're right, there were
several manufacturers that made these units (Midland, Pace, Numark,
etc.). I did try to hook it up, but the SWR was 1.3:1 (almost the same
as I got with a dummy load), which I thought was too good to be true,
but maybe it was due to the low wattage being used (4 watts). I'll get
some proper cables built and go from there.

Joseph2010-

Ralph is right on the money with his advice. I've had a couple of the
CB-type bridges that worked well on Two Meters.

I think your 1.3:1 measurement may be due to mismatch of the 150 Ohm
resistors in the bridge. But it also could be telling you the truth!
Some dummy loads are not good on Two Meters.

Hooking it up backwards should work the same as forward, except the
switch works backwards.

You normally adjust an antenna for best SWR assuming the meter is
telling the truth. In fact, 1.3:1 is not so bad. If you did not have
that dummy load, you might have adjusted the antenna for a reading of
1:1 and never found out it was not perfect.

Fred
K4DII