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Old 23rd Jan 2020, 5:09 pm   #1
Andrew B
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Default SWR Meter

Hello everyone.

I have an SWR meter for HF 1-30MHz that I would like to use / adapt for 70cm use. The meter is a not too expensive one and I wondered what the differences were between an HF and a UHF SWR meter?

Thanks in anticipation
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Old 23rd Jan 2020, 5:51 pm   #2
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Default Re: SWR Meter

I would imagine that the 70cm one will use a very short length of the through signal path to "sniff" the RF in either direction, compared with an HF one.

Cheers

Aub
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Old 23rd Jan 2020, 6:32 pm   #3
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Default Re: SWR Meter

The 70cm sampling head will use short, coupled transmission lines and a couple of diode detectors. One line carries the signal, the other has a sample of the forwards and reverse powers induced in it. The detectors sense these induced powers. the meter, switch and set pot let you compare these powers.

To work the same way, an HF coupled line arrangement would have to be 433 times longer! This isn't feasible. So the HF VSWR meter uses transformer coupling to sense the current in the antenna, and usually uses a capacitive attenuator to sense the voltage. There are some coupled line meters for HF (Eg the right slug for a Bird 43) but they are limited to very high power ranges because the sensitivity is lousy with the short lines,

Trying to use a regular HF VSWR meter at 433MHz won't work because the ferrite core material will have become terribly lossy at that frequency

David
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Old 23rd Jan 2020, 10:50 pm   #4
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Default Re: SWR Meter

While you can probably re-cycle the box, the meter, switches and connectors, that may be all about all. But you should find any number of construction articles for a UHF meter via Google (or RSGB / ARRL handbooks) and the coupling line required may be something as simple as two parallel tracks of copper on a PCB board, provided you're not using too much power.

Quite a lot of SWR meters intended for use up to 30MHz were probably sold on to the CB market, and were "cheap and cheerful".

B
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Old 24th Jan 2020, 5:02 am   #5
John KC0G
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Default Re: SWR Meter

Phil Salas, AD5X, published an article "Accurate Low Cost VSWR Meter - Convert this CB accessory for 1.8-450 MHz operation" on pages 18-19 of the February 1993 issue of 73 magazine. You can find the magazine issue at https://ia800607.us.archive.org/23/i...ruary_1993.pdf
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Old 24th Jan 2020, 4:41 pm   #6
Andrew B
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Default Re: SWR Meter

Thank you for all of your helpful replies. It looks like a cheap ex-cb one, it has the box, meter, switch etc, so that's all the main (expensive) parts. I'm only using a Baofeng UV 5 to acccess the local repeater (GB3HD) for the weekly club net GX2UG on wednesdays at 7pm.

Thanks everyone.
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Old 24th Jan 2020, 7:42 pm   #7
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Default Re: SWR Meter

The (hopefully) attached PDF found online describes conversion of an ex-CB SWR meter to UHF usage by replacing the original works with something aimed at UHF. As I'm not really an RF person I can't speak for how good or bad the approach adopted is, but I'm sure folks who know more will be happy to take a view.
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File Type: pdf uWave PSV metr_EN.pdf (663.8 KB, 45 views)
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Old 24th Jan 2020, 8:40 pm   #8
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Default Re: SWR Meter

That looks like a good approach. The difficulty in building one will lie in calibrating it afterwards if you don't have trusted instruments available for comparison.

My preference with these things is to forget the VSWR set pot and just have two meters, one showing forwards power, the other reverse power. Best match is with reverse power dipped. Power amps will be happy if the reverse power is below some level. You can take the ratio of the powers and get the return loss of your antenna/feeder and if you want, calculate VSWR from that.

Hving two meters is good. Having one meter condemns you to keep flicking a switch backwards and forwards for eternity.

David
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Old 26th Jan 2020, 12:09 am   #9
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Default Re: SWR Meter

Both of the modified SWR meters look a bit dodgy to me but I guess a lot depends on what is expected or needed from such a meter up at 432MHz.

The classic (passive) CB SWR meter of the last 40 years or so is based on quite an elegant design in my opinion. However, it is really easy to pour in shovelfuls of measurement uncertainty if an attempt is made to modify it up to UHF.

The OK1TIC swr meter uses a coax coupler so it should have low through loss and low through VSWR at 432MHz but the coupler/detector response won't be the same across all three bands. The original CB meter scaling will presumably be designed for a classic linear detector (or something fairly close) The OK1TIC meter won't be faithful to this scaling especially when operated across the three bands it was designed for and at various transmit power levels. This is because of the variations in detector response for this type of small signal diode at small signals. It might have been better to describe it as a mono band meter that operates over a fairly restricted power range, but even then the linear scaling of the original dial will be an issue here I think.

A ballpark guesstimate would be that the meter could have an VSWR measurement uncertainty spanning something like 1.3:1 to 2.3:1 when measuring a genuine 1.7:1 VSWR. However, I feel I may be a bit optimistic. A lot depends on how much the detector response fails to track the dial scaling as this will further degrade the uncertainty associated with a 24dB directivity.

The overall measurement uncertainty caused by this (and the 24dB directivity) will place this meter in the 'adequate' class despite the author's original design goal of producing something intended to be quite accurate.
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Old 27th Jan 2020, 11:05 pm   #10
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Default Re: SWR Meter

Here's a few observations about the OK1TIC SWR meter. I've written it in a hurry but I think most of it is OK.

The coupler has a single coupled arm with a terminated detector at both ends. This will become troublesome at UHF because the capacitance of the detector diode pair will begin to degrade the return loss of the termination resistors. Even 2 x 0.25pF = 0.5pF across a 50R termination can be significant up at 432MHz. This will degrade the directivity and I assume that the coupler directivity was measured on the VNA with the detector still fitted at one end rather than a SMA termination. This might explain the relatively poor directivity of this coax coupler up at UHF.

A CB SWR meter design typically tries to get lots of RF voltage to the detector diode (to achieve linear detection across a wide range of RF power levels) so it will use fairly tight coupling and it will also leave the detector end of each coupled line unterminated to get a voltage doubling and it will typically use a 150R sampling line. All these things boost the RF voltage at the detector diode and this makes it easier to achieve a 'linear' RF voltage detector using a typical detector diode. These things are important for a meter that has to work with lowish power levels of about 4W.

With a linear detector the VSWR scale becomes self calibrating as it is really just a [mag]reflection coefficient meter marked up as VSWR. So halfway on the dial will be a coefficient of 0.5 = VSWR 3:1.

VSWR = (1+r)/(1-r) where r is the magnitude of the reflection coefficient so it should be easy to plot the rest of the VSWR scale on a meter like this as long as the detector response is linear. I presume that's what many CB meters do although the very lowest part of the dial may have to account for a small amount of detector non linearity especially at the 4W power level of a CB. For example a VSWR of 2:1 is a reflection coefficient of 0.3333 as r = (VSWR-1)/VSWR+1). So the VSWR meter pip for 2:1 VSWR would be 1/3rd of the way up the dial.

It's worth having a look at the scaling for your VSWR meter to see if it conforms to the above linear scaling. Often the 2:1 pip is slightly lower than 1/3rd deflection and this accounts for some detector non linearity and inefficiency. If it is higher than 1/3rd of the way up the scale then that would be a bit odd but I have seen CB SWR meters that are like this!

It's possible to predict the uncertainty window of a coupler with 24dB directivity with a few equations. The window for a 1.7:1 VSWR calculates to be 1.48:1 to 1.98:1. So the indicated VSWR could be expected to vary across 1.48:1 to 1.98:1 as the angle of the reflection coefficient is rotated around a 1.7:1 VSWR circle on the smith chart. That level of performance is probably adequate for most hams. However, there will be other errors due to the non linear detector response as below.

The coupling factor at 432MHz is about 35dB. So presumably a transmit power of 31.6W or 45dBm will mean that the detector gets presented with +10dBm and this sets the FSD 'cal' of the SWR meter. If the meter then tried to measure a return loss of 15dB (VSWR about 1.45:1 and r = 0.183 ) then the data in the OK1TIC pdf article shows a raw detector voltage ratio of 1434/150 across +10dBm to -5dBm. This is a ratio of 19.6dB (r = 0.105) and not the desired 15dB. So this detector is definitely not linear...

So this adds to the uncertainty above because a typical CB VSWR scale will still be fairly close to a linear response at this VSWR reading. It would be even worse if the test was done at 10W power as the ratio is 725/59 = 21.8dB (r = 0.081). This shows how the errors mount up with this meter even with a small change in power level. The errors can compound with the uncertainty due to directivity so these errors could become significant for certain parts of a 1.5:1 VSWR circle on a smith chart for example.

I think the diode used was a HSMS-2855 dual diode and this diode is easy to damage and it is not really designed for linear operation.

None of the above may matter to most hams and it probably doesn't matter if the VSWR reading is a bit off. It would only matter for a high power solid state PA that might fold back the power if presented with a VSWR over 2:1 for example. This might cause some confusion if the meter reading was falsely showing a VSWR of 1.5:1 for a genuine 2:1 VSWR.
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Last edited by G0HZU_JMR; 27th Jan 2020 at 11:11 pm.
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Old 28th Jan 2020, 1:27 am   #11
Andrew B
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Default Re: SWR Meter

Dear Jeremy, thank you very much for the deep explanation of the difference in meters and for taking the time to type it all out for me.

Andrew
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