SWR meter

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Crash one
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SWR meter

Post by Crash one » 10 Jun 2017, 01:44

I've just bought a SWR/power meter. Zegati 3--200 MHz to check a handheld on airband 118--136. It says in the leaflet that watts is only accurate in the 26--30 Mhz.
Is this normal? How inaccurate is it likely to be? Anyone know of the brand? Working blind here, have I made a mistake?

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Re: SWR meter

Post by paulears » 10 Jun 2017, 07:30

Yep it's probably accurate - but only in that band range. Cheap meters have diodes that are only linear and matched in certain ranges. This is why expensive accurate meters like birds use just one and it can be physically rotated. It's quite possible that at air band the two diodes are so far apart that they indicate totally wrong information and using an oldie of mine as an example, they cannot even indicate a full scale on transmit!

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Re: SWR meter

Post by Crash one » 10 Jun 2017, 10:34

Thanks! You live and learn. I don't understand what MHz has to do with Watts, two different people innit?

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Re: SWR meter

Post by oh5nxo » 10 Jun 2017, 13:41

To measure watts, forward & reflected, the meter uses capacitors & coils, or short lengths of parallel lines, which work like caps & coils. It's hard if not impossible to find components that would work as intended over a wide range.

Juha

There's a junk box special for reliable power meter VLF ... UHF. Just power, no SWR. Female BNC, 50ohm 30W resistor, 1N4148 diode, 1nF feedthrough cap in a suitable metal plate. Read DC voltage with multimeter, square, divide by 100 to get watts. The diode pops at around 50W, so make it easily accessible :)

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Re: SWR meter

Post by paulears » 10 Jun 2017, 15:50

The MHz and Watts thing can be explained away fairly simply - forward power is done by using the diode to cut out anything going one direction, remembering that the actual RF is AC - so using a diode strips out 50 percent of it, leaving just the top positive going portion of the waveform. Then in a cheap meter, there's another (or the same one reversed by a switch) that changes it to removing the other half, the negative going portion - The trouble is that diodes that work well with DC by design, are less good with AC when the frequency goes up - so 27 million a second they can pass OK, but 100 million and they struggle. Sometimes the struggle is so bad that very little gets through at all. As many VSWR meters use the meter to indicate power going through - on the principle that if 10W goes out, and none comes back you have a good VSWR and full deflection on the meter can be marked as 10W. Stick the 27MHz meter on 430MHz and that meter might not even display ANY power getting through - even when it is. If the diodes can't hack it - the meter's a bit pointless.

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Re: SWR meter

Post by Crash one » 11 Jun 2017, 10:46

Well at least I've learned that when I get this aerial built and the connectors arrive and I try to test it I'll know that a rubbish reading isn't necessarily down to my workmanship.
As long as I can tune the thing, which is why I got the meter in the first place?
Thanks gents, that's £45 I'll never see again!!

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Re: SWR meter

Post by Admiral » 11 Jun 2017, 14:24

I trust you have a licence to TX on those frequencies? We can mess about all we want on 446 and get ignored, but any unlicensed RF on that frequency will get you in a lot of hot water.
Winner of the 2017 IBTL 'Summer Sizzler' competition

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Re: SWR meter

Post by Crash one » 12 Jun 2017, 10:28

Yes I have a Flight radio transmitting operators licence or FRTOL. CAA requirement to use radio in flight.
Next problem, I have discovered that although the dipole would fit into the aircraft I would need arms four feet long with three elbows and six fingers to actually reach to fit the thing.
Plan B. Build a standard monopole whip with a ground plane which will be reachable.
Back to the drawing board!

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Re: SWR meter

Post by Admiral » 12 Jun 2017, 12:28

I had a licence once upon a time as I worked airside at many sites as a metallurgist, the irony being that I never touched a radio in all my time airside.
Have you considered a bluetooth connection to the set?
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oh5nxo
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Re: SWR meter

Post by oh5nxo » 12 Jun 2017, 12:35

The plane was non-metallic? How about copper tape on the surface?

Endless source of ... ideas.
Juha

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Re: SWR meter

Post by Crash one » 12 Jun 2017, 18:25

oh5nxo wrote:The plane was non-metallic? How about copper tape on the surface?

Endless source of ... ideas.
Juha
Copper tape could be a plan.

Endless source of .... Questions.

1: I don't have a width of half wave for the ground plane?
2: do radiating ground plane wires/tubes have to be equi-spaced round the circle?
3: do they all have to be a quarter wave long?
4: there is a hefty chunk of engine about 6ft in front of where I want the aerial to be?
5: a steel bottle fire extinguisher a few inches above where the whip will be?
6: could the whip be fitted upside down, ground plane copper tape on top, or horizontal?
7: What would be a "good" SWR reading?

This is an interesting subject, with either a simple solution or a masters degree in radio science.
Reading about antenna theory, not understanding the symbols and trying to pick out useful bits. Difficult.

oh5nxo
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Re: SWR meter

Post by oh5nxo » 12 Jun 2017, 20:04

The engine and all nearby large objects will have an effect on the antenna. Large meaning larger than a quarter wave. If you are really unlucky, there will be nasty nulls in the pattern, but first approximation, don't worry yet. Most likely you'll have a reduced range in some directions, enhanced in some others.

You can be creative with the ground plane. Just two radials, forward and backward should be ok. Or a narrow X if it fits. Again, the pattern will be distorted, but not dramatically so.

Upside down vertical whip will be good, I'm guessing. Horizontally you'd have noticeable deep nulls in two compass directions. Vertically, the nulls are into space and into ground. When you are just above a ground station, the distance is so small that the null does not matter.

SWR is overrated. Below 2:1 is fine, unless you have a picky transmitter. A handheld should not be so fussy. You'll have no trouble finding 1.5.

BTW, the tips of a dipole can be bent a bit with little effect. Could you fit a C-shape vertical dipole on the side, like half-painted invasion stripes ?

Juha

EDIT: Uh, I'm stupid, of course the antenna has to match the other end and be vertical.

Can there be some kind of microphone AGC issue? If there's loud ambient local noise, some radios might adjust the mike gain down.

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