Differing SWR readings.

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sureshot
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Differing SWR readings.

Post by sureshot » 01 May 2018, 00:35

Recently picked up three small linear amplifiers, they where going for not a lot so took a punt on it.
Having got these amps, they needed new power leads and a clean up. Also renewed the thermal compound after testing the transistors.
All three scrubbed up well, they are a CTE 737, Midland 747, and a CTE 767.
Two out of the three perform as expected, SWR is under 1.1.5 the CTE 767 and Midland 747 are the trouble free amps.
The CTE 737 is being a bit difficult, the SWR with that in line won't come under 1.2.0 this is with a 4 watt drive, it's marginally better with a 2 watt drive. I had noticed no relay in the little CTE 737, so guess it's solid state switching. It does SSB as well. So I'm at a loss as to why the SWR won't come down with it.
It's drawing the expected current with a multimeter in series. I've only got the one power meter at the moment, and that's keeping an eye on the input drive power. I don't think I will need much more than a 2 watt drive with any of them for my needs. Just can't work out why the SWR on the CTE 737 won't come down lower.
Thoughts and ideas appreciated. :)
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Re: Differing SWR readings.

Post by paulears » 01 May 2018, 07:29

The solid state switching introduces mismatches in the system and at the price point won't be as effective as a real relay, however I don't think it's perhaps as bad as you think - 1:1.5 is OK in real terms, and 1:2 only a bit worse. (The colon is important in VSWR - at first I thought you meant 1.2, not 2)

One thing - have you used a back to back coupler instead of the amp, and checked the VSWR without the amp in circuit? It's possible your antenna isn't quite resonant, and the solid state switching is making the matching worse if the antenna isn't quite there either. Have you checked that on receive, the solid state one is working fine - as in not having losses too? Worth checking, because if there is a proper TX problem, there could be one on RX too? The other thing could simply be your meter. the VSWR on 2W or 4W drive should not change. If it does this usually is non-linearity in the meter diodes - so it's even possible your meter could be joining in making things appear worse than they are?

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Re: Differing SWR readings.

Post by sureshot » 01 May 2018, 09:52

Thanks for the help, my meter is a cheap CTE SWR and PWR meter. I could do with getting something a bit better quality. Yes i did mean SWR of 2.0 and not 1.2 sorry if i got that bit wrong.
The SWR changes even with a straight radio SWR meter hook up. I've had to add a matcher as the SWR won't come down below 2.0 with out it. I get 1.1 to 1.2 with the matcher at 4 watts. That goes up if i increase power from the radio. At full power approximately 12 watts FM its an SWR of 2.0 with the matcher. As I've only got a balcony it has to be a mobile antenna, I've tried a solarcron tri load, a mini springer, and now a Sirio 4000.
Very similar results with all three antenna, amplifiers aside, the SWR goes up with increases in power. 4 watts doesn't look to bad, but radio only 12 watts is around or just over an SWR of 2.0 .
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Re: Differing SWR readings.

Post by paulears » 01 May 2018, 20:43

VSWR is a ratio - power has no impact on it at all. Half a Watt or 10W into the same antenna should give identical readings. Let's say you have 1W, and 10% is reflected, 1W out, .1W returned. Turn up to 10W and you get 1W back - the antenna doesn't behave differently if you stick more power up it! If you turn up and it changes, the meter is faulty, or probably just not very accurate. If you are basing your tweaking on a dodgy meter - you can't rely on the results. If the 4W reading is OK, I personally, would ignore the higher reading as likely to be the dodgy one.

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Re: Differing SWR readings.

Post by sureshot » 02 May 2018, 18:44

paulears wrote:
01 May 2018, 20:43
VSWR is a ratio - power has no impact on it at all. Half a Watt or 10W into the same antenna should give identical readings. Let's say you have 1W, and 10% is reflected, 1W out, .1W returned. Turn up to 10W and you get 1W back - the antenna doesn't behave differently if you stick more power up it! If you turn up and it changes, the meter is faulty, or probably just not very accurate. If you are basing your tweaking on a dodgy meter - you can't rely on the results. If the 4W reading is OK, I personally, would ignore the higher reading as likely to be the dodgy one.
I do recollect having read the scenario you describe above, and it did have me a bit baffled as to why thing look different with power increase. I though it might have been something to do with the matcher, as the way the impedance is seen by the meter. Yes it is a cheap and cheerful meter, although readings seem consistent across three radios and amplifiers. It's just the CTE 737 that doesn't give the same, or similar readings.
Another day I'll have to put it inline again and see if I can do better, the evening I tried it cables where a bit how ya doing. So might have thrown the results.
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Re: Differing SWR readings.

Post by paulears » 03 May 2018, 07:25

Any power increase changing readings points to the meter, because a ratio is a ratio - power doesn't impact on it at all.

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Re: Differing SWR readings.

Post by The Collector » 04 May 2018, 11:23

As Paulears said, probably an inaccurate meter. I had one like that years ago and it'd change on 4w/ 0.4w power output, let alone anything drastic.

Get your leads as short as is possible with as few connections/joins as possible too. Use the larger co-ax if possible, although this isn't always practical where SWR meters etc are concerned and situated. Don't coil any excess co-ax either. Check then for any continuity/shorts before throwing them into your set-up.

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Re: Differing SWR readings.

Post by sureshot » 05 May 2018, 10:25

The Collector wrote:
04 May 2018, 11:23
As Paulears said, probably an inaccurate meter. I had one like that years ago and it'd change on 4w/ 0.4w power output, let alone anything drastic.

Get your leads as short as is possible with as few connections/joins as possible too. Use the larger co-ax if possible, although this isn't always practical where SWR meters etc are concerned and situated. Don't coil any excess co-ax either. Check then for any continuity/shorts before throwing them into your set-up.
Yes i will keep leads short, and will invest in a better quality meter and see what that says.
With all three amps i am putting through a matcher. Two amps don't require and adjustment but the CTE 737 does to get the SWR even out of the red. Strange one, but i will get round to getting a better SWR PWR meter. Thanks for the help guys.
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Re: Differing SWR readings.

Post by Kaliphan » 08 May 2018, 11:08

How is it all connected?

Rig, meter, amp, matcher, antenna or some other way??

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Re: Differing SWR readings.

Post by sureshot » 17 May 2018, 10:05

Kaliphan wrote:
08 May 2018, 11:08
How is it all connected?

Rig, meter, amp, matcher, antenna or some other way??
Same as the order you have stated.
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Re: Differing SWR readings.

Post by Kaliphan » 18 May 2018, 14:43

First off, the 737 isn't solid state or relay switching, it uses that crappy push switch to conect the amp into circuit, it's a cheap rubbish amp best put in the bin.

Second, with your meter there on a good amp all you're measuring is the SWR into the amp which shouldn't be affected by the matcher unless the amp is turned off, because you can adjust it.

What you might be seeing is a whole load of RF leakage back through that crappy switch and your meter is measuring that and shows it up as a load of reverse power which it shows as bad SWR.

It should be connected

Rig Amp SWR Matcher Antenna

because that means you're measuring the tuning of the matcher but it will also tell you if your amp is oscillating on some random frequency because the SWR will shoot up and you won't be able to tame it.

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Re: Differing SWR readings.

Post by Kaliphan » 21 May 2018, 08:59

Kaliphan wrote:
18 May 2018, 14:43
First off, the 737 isn't solid state or relay switching, it uses that crappy push switch to conect the amp into circuit, it's a cheap rubbish amp best put in the bin.
should be "isn't full solid state"

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