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V SWR - Measure on a Meter or on the Rig?
Topic Started: Tuesday, 24. November 2009, 22:13 (864 Views)
VoG
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A couple of people have asked why, or how, a SWR meter can sometimes give a difference reading to an in-built SWR Meter..... I’m no huge expert on this, but I have a few thoughts. If someone knows better, feel free to correct me! : )

Firstly, not many “popular” SWR meters can claim to be Precision Instruments. The most common sort, about three inches by two (or 7cm by 5cm for the children here) are, frankly, pretty poor and really only serve to detect and warn of Really Bad SWR. I wouldn’t trust them to give me an accurate reading at all, but telling me whether an antenna is safe to TX into, or is likely to blow my finals, is a fair use of them.

Another area of doubt is the way many SWR meters work. You have to set them to FWD or CAL and then adjust until the needle hits the SET mark, or is it the Red Mark, or is it somewhere in that Red Band?

Then you have to switch to REF or SWR and read the result.

But do you let go of the PTT while you switch over then re-key, or should you keep the PTT pressed while you switch to REF, or SWR?

All of that can be overcome with a cross-needle SWR. Press TX and where the needles cross is the SWR of that antenna. OK, might be some mechanical inaccuracies in the meter, but at least the basic human error has been removed.

One other thing, the SWR is all about the antenna. You’re measuring the reflected radio from the antenna at a given frequency, specifically the Voltage Stand Wave Ratio. So the radio, the mode and the power doesn’t matter. You should be able to put a 4w FM rig on an antenna and check the VSWR and it should be the same for a 50w AM set, or a 200w USB rig. Usually we check SWR with a fairly low power (10w max) in case it is awful. So, if 90% of the power is reflected back into the box, I’d personally rather have 90% of 10w coming back at me than 90% of 1Kw – guess which is more likely to fry the finals?

If I had an external meter that was of decent quality, then I’d probably trust that first as it is built and designed for the job. The internal SWR meter on many older style rigs (Cobras and 3900s) uses the little TX/RX/SWR dial to show the reading and it isn’t easy to read, and I don’t think that dial is much of a precision meter either.

But, also remember that if you’ve just taken the SWR on an external meter, and then you’ve done the same with the internal meter, you’re not always comparing like for like. The internal meter is just checking the SWR down the cable, onto the antenna, and back.
The external has to use a patch lead and two extra PL259 plugs, plus some unshielded wire inside the meter, so there’s some extra “baggage” there. Or, worse of all, if you use the internal meter to check the SWR while you have the external meter is still connected, you have the worst of both worlds.



Back to the initial question: if an external SWR meter gives a different reading to an internal meter, which do you believe?

If I have a quality meter, preferably cross-needle, the external anytime.
If my external is a cheapish “CB” type meter, and my patch leads have seen better days, then the internal is probably more trustworthy.



Another, unproven but commonly held, belief . . . SuperStar 3900s like as little as possible between them and the aerial… For some reason they seem to have a high error rate when anything extra (SWR meter, low-pass/TVI filter, etc.) is put inline between the rig and the antenna. Don’t know if it’s true, don’t know why, but I;ve heard it a few times.

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Firetrap
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26CT100

I found on the 3900 that the external meter reading was lower than that of the built-in meter, however, the external one was no more than £15 and use for an approximate reading is about it, With advice from Cherokee i always stuck to the built in meter and i never had any issues!

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