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Author Topic: Using standard "HF" SWR meter on 2m...  (Read 1845 times)
KC0LPV
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« on: September 06, 2003, 10:55:47 PM »


I have noticed that my Radio Shack Field Strength/SWR meter does "seem" to work on 2m.  I can calibrate it and it seems to give me an SWR reading that makes sense (a ground plane 1/4 wave is around 1.2:1, for instance).

Can I use a meter like this for 2m?  Meters labled for VHF seem to be fairly expensive, and I can't justify spending the money if I can make do with what I already have.  Student life, you know.

Others in the area have similar meters, and the only guy I know with an MFJ Antenna Analyser is a hundred miles away.

Advice?

Jim kc0lpv
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KE4MOB
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« Reply #1 on: September 06, 2003, 11:06:53 PM »

I know this is really isn't an answer to your question, but check out a Struthers AN/URM-120A wattemeter on eBay.  All bands (2 Mhz to 1 Ghz) with the three slugs) for 80-100 dollars!!

Look around, there's lots of used stuff to pick and choose from.

Steve, KE4MOB
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K0IPG
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Posts: 252




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« Reply #2 on: September 06, 2003, 11:13:22 PM »

I'm sure there will be those who disagree with me on this one, but I'd say go ahead and use it - it's reasonably likely that it'll be accurate enough for your purposes. You could easily test it's accuracy by connecting a few combinations of resistors to it (ie 150 ohms should show 3:1 swr, 50 ohms should show 1:1 swr, etc - make sure to use non-reactive resistors, though).
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W4TYU
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« Reply #3 on: September 07, 2003, 06:46:01 AM »

If you can set the meter to read full scale when on the "direct" position, It is usable and as accurate as many meters.

However, be aware that to get "good" readings on VHF the SWR meter should be as close to the antenna as possible. This is because the feedline loss works in both directions.
For  100 feet of RG58 at 2 meters, 3db forward and 3db reverse. The decreased reverse signal will lead you to believe that the SWR is lower than it actually is.

Ole man JEAN
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KD7EZE
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Posts: 60




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« Reply #4 on: September 07, 2003, 08:40:03 AM »

You can use it, but anything that you read on it will be wrong. Alot of people ask this question, mainly trying to save a buck or two. An antenna analyzer is nice, but not necessary. 2 meter SWR meters are not that expensive new, and can be had fairly cheap at hamfests and internet classifieds. Besides, an SWR meter designed for the band you intend to use it for, is cheaper than a new rig, which is likely to be damaged, if you are trusting the readings of a meter not designed for the band of the radio you are using.

KD7EZE  
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W4TYU
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« Reply #5 on: September 07, 2003, 11:31:39 AM »

Why will everything read on the meter be WRONG ?  What is different in the design of a VHV SWR meter from that of a HF SWR meter,

Yes I have kept a meter in line but only to monitor the reflected power which is where a problem shows up.

The dense old man  JEAN
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AB0RE
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Posts: 293




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« Reply #6 on: September 07, 2003, 05:24:24 PM »

I have used Radio Shack's CB SWR meter for 2M and the readings seemed to be fairly reliable.  I remember reading somewhere (one of the new ham study guides, maybe?) that sometimes this can be done if TX power is kept low.  You can use the meter to determine what antenna changes increase/decrease SWR, but I'd suspect that a 1.2:1 reading might not necessarily be 1.2:1 on the nose.

If you do decide to go with a meter specifically designed for VHF/UHF, I'd suggest the Diamond (non-mobile) unit.  I'd stay away from the MFJ 812B - I had one of those and it never did work, despite being sent in for service twice.

73,
Dan / ab0re
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WA4PTZ
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Posts: 528




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« Reply #7 on: September 08, 2003, 05:27:48 AM »

It all revolves around the type of diode and other
components used . For HF a 1n34 is often used which
cannot handle RF above about 30 MHZ , the 1n60 can
can handle higher frequencies although I don't recall
how high. You can find a suitable diode with a little
research. Now for the other components....
the capacitors may need to be changed to silver mica
type, but before you change them try it out . Be sure
that you have a "known good" meter to compare the
readings. Oh, and keep the component leads as short
as possible.
73 - Tim
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KB0ETC
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Posts: 248




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« Reply #8 on: September 08, 2003, 05:58:40 AM »

Words I don't agree with... "Radio Shack" and "Reliable" at least in the same thread.

Look for one designed for your specific application, is it really worth taking the "gamble"?

Although it may work, it will occur sooner or later when a failure happens... i.e. meter, radio, etc...

Just like everything else use only for the intended application.

I have Diamond meters that have multiple sensors for different bands.

73's
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W8JI
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« Reply #9 on: September 08, 2003, 06:00:16 AM »

The VHF readings could be wrong or unreliable, or could be OK. The only way to know would be to use several loads and test it, or compare it to a known good meter in several situations of known SWR where the load is both high and low in impedance. It might read accuratelty with high impedance, and not good at all with low, or any other combination!  

There is a sigificant difference in design of many HF meters and VHF meters. The design change centers  around the pick-up system or directional coupler. In a Bird meter and other commercial meters, they actually change the pick up element with frequency range. Even doing that, the accuracy is + - 5% of full scale rating anywhere on the scale. Most lower frequency designs use ferrite cores, while higher frequency use strip (transmission) lines.

The readings also could be OK, but I'd be a little skeptical without checking. If the meter worked even reasonably close to specification on VHF, you can bet the manufacturer would have advertised it to do that!!!

Very few manufacturers would spend a lot of time designing something and then "under rate" it for frequency coverage just to loose potential sales!

73 Tom
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WB6BYU
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Posts: 13580




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« Reply #10 on: September 08, 2003, 04:19:48 PM »

Certainly you can use it - you already have, since you
say it reports reasonable readings.

The real question is whether it will work for your
needs.  If all you want is some reassurance that your
SWR is "low" or "high", then it may be adquate.  If you
want a precision reading of output power then it is not
the right tool for the job.

I have several that I have used for tuning up antennas.
Usually I am adjusting something for minimum SWR:  the
indicated reading is not necessarily very accurate,
but if the indicated reading is "low", perhaps under
1.2 : 1 or so, then the actual SWR is probably below
2 : 1 and the rig is not in danger.

But if it makes a difference to you whether the SWR is
1.8 : 1 or 2.2 : 1, this is not the right instrument.

Actually I've had more problems using such meters on
80 and 160m than on 2m:  the meters are frequency
sensitive, and require more power to drive them to
full scale readings on the lower bands.  On 160m I
couldn't get enough power out of the rig to measure
the SWR unless the SWR was low to start with.  This
makes it VERY difficult to adjust a tuner!
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