Radio Mysteries

Here’s three mysteries I can’t get my head round, and even had @G3CWI shrugging his shoulders.

  1. When operating PSK or RTTY, it is really hard not to suffer RF feedback issues. I’ve only been able to reduce this either by winding a choke balun at the feedpoint, or adding a long extension to the feeder cable in order to have the interface well away from the antenna, including radials. With EXACTLY the same set-up, FT8 causes no problem at all. Why?

  2. By having my FT-817 meter set to PW (power), I can see that after switch-on, power output on HF bands is very low. After a few minutes of operating (choice of mode makes no difference), the output power starts to creep up, and after 20-30 minutes, it’s getting towards full power. This is not the case on 50MHz and above, where full power is indicated from the off. Is this a finals issue? But they do work - eventually…

Both 1. and 2. above are noted on a variety of different antennas.

  1. I updated WSJT-X to the latest version today. Suddenly I now seem only able to decode incoming signals that are 200-700Hz below mine. Nothing has changed w.r.t. RX filtering into my set-up but I do note some mention of “bandpass” in the list of changes to this latest version - probably need to investigate!

Are you sure that no-one has put a curse on you?

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If you can get full O/P on 50MHz up from cold then the finals are fine.

Something changing after some time using it says there is a temperature issue. Leave 817 on/by a radiator at home to heat it up for 1hr then check output power.

It could be the ALC is reading too much and turning down O/P until ALC is warm. I haven’t looked at if there are HF and VHF power sampling circuits on the 817 or if the same is used for both.

Only a guess, but are you operating RTTY/PSK through the data port, and FT8 via the normal speech channel? Could be a difference in the filtering…

I’ve noticed (and been alarmed by!) this phenomenon with my '817s on several occasions, always when first setting up out portable. Did wonder whether it could be due to condensation on a PCB altering the bias conditions somewhere in the transmit signal chain. If so it’s fortunate the 817 runs nice and hot to dry it out!

I have also had this feel on HF with my FT-817, but I haven’t investigated it deeper, as it’s always given me so very good results and lots of satisfactions. When I first noticed this at the begining of my SOTA activating, I put it down to the SLAB battery I was using at that time. Later I realised it wasn’t the battery and I concluded it was related to the FT-817 temperature. Since I built it, I have always had my FT-817 inside a wooden box and I think this helps it to warm up soon after having started to transmit. Fortunately, I haven’t noticed any long time to be chased or spotted by RBNHole after my transmission start.
Whatever it is, I don’t give it much importance, as it’s a quickly overcome situation.


A sudden change to radiating full power would suggest feedback but if the problem is slowly apparent as the set “warms up” it sounds much more like the balanced mixer (often an op amp device these days) is drifting and losing carrier balance. Is this a common fault?


Sounds like the PA driver stage to me.
Doubt there is any closed loop control as the PA bandwidth is very wide in the FT817.

You’ll have more gain there then in the finals and hence the large power variation between first switching on and stable.

This gives a good technical description of how the OP power is calibrated for the FT817

Perhaps recalibrate your TX gain (Q1007) use in colder temps ? Apparently it’s quite sensitive. Put the radio in the freezer for a while and then attempt a cal. Some of us have environment chambers for this purpose at work :sweat_smile:

Just some ideas


“By having my FT-817 meter set to PW (power), I can see that after switch-on, power output on HF”

Just to be different my 817ND behaves like this on 2m while at the same time being fine on both HF and 70cms. It’s an odd one.


My two penneth Tom,
Point 1 - how far apart are the frequencies between PSK/RTTY and FT8? Could it be that the antenna is off-tune on one frequency and OK on the other?

Point 2 - could this be in some way also antenna related and the SWR protection is folding back the power? But why does the antenna get better over time (and hence the output power increases)?

I’d suggest you see if these conditions occur into a dummy load - Oh and another thought - could lt be that one of your connecting co-ax cables is exactly a 1/4 wavelength (or multiple thereof) long? I’ve seen that cause some really weird problems in the past.

Are both these problems occuring on multiple HF bands (I know they dont occur on 50MHz) ?

Have you tried the FT-817 on different HF antennas?

73 Ed.

19" long length of coax-patch cable?

It doesn’t.

Hi Ed,

Certainly what I have observed is not antenna related - I’ve only noted it three or four times in several hundred activations with the same antenna (or rather it’s many clones!) I use an external ATU with power and SWR indication, you can tune up fine on the one or so watts the radio will produce and over the next ten minutes or so the power will gradually increase to maximum without further need for ATU adjustment.

I have never ever noticed this in any other circumstance than on first setting up when out portable.

Hi Paul,
agreed in your cases with an external antenna matching unit no matter which of the various antenna “clones” you use shouldn’t be relevant as the rig should (hopefully) always see a good SWR all the time as it sees the input to the ACU and so fold back of the output transistors because of bad SWR shouldn’t occur.

The fact that this increase in power occurs over time would suggest something is changing. Perhaps component temperature related. Since you have only seen it three or four times in several hundred activations, is it possible that these were days where it was particularly cold? We’re you perhaps running off an internal battery pack and as the temperature of the rig rose through usage, the batteries warmed up to operating temperature?

It’s an idea in any case.

I wonder - has anyone checked the FT-817 forums looking for these symptoms and reports from other owners?

73 Ed.

Hi Ed,

Yes that’s my point… the rig still only produces 1w or so at first, despite the match being OK via the ATU. No further adjustment of the ATU is required as the power rises.

No, don’t think so - unfortunately the incidents are so infrequent and the last occurrence so long ago I don’t recall the prevailing conditions. But I do know for sure I’ve done several very cold weather activations at temperatures down to -10C when the issue hasn’t arisen. The 2200mAh LiPos I use seem well able to adequately power the '817 at these temperatures. I operate with the battery on top of the radio so it’s soon toasty warm anyway :slight_smile:

So rare I’m not going to worry about it, but when (if?) it happens again I’ll take more note of the circumstances!

73 Paul G4MD

Good point about the power calibration. The four power levels each have a tx gain setting and an output limiting setting. If the power level in question was calibrated quickly by setting the output limit (using ALC feedback) and left the tx gain too high, there is potential for drift in the output power monitoring to allow the power to rise, due to having tx gain set too high.

Without sufficient tx gain the output power cannot rise. If the power is rising, the gain is too high. I suggest trying other power level settings. Eg instead of using 1w, use either of the levels above or below that. Just to test whether the same power drift problem occurs.

73 Andrew VK1DA/VK2UH


Hi Paul,
I no longer have my FT817ND, so I can’t see if I can create these problems, but from what you say battery temperature is probably not the problem.

However I do still have two used 2500mAH lipos with compatible cables that fit inside the FT-817 battery bay. Problem is that it appears there’s no way to ship LIPO batteries out of Germany any more - DHL bounced my attempt to send them to a Ham in Australia. - If there are any SOTA operators in Germany who want a LIPO battery for their FT817, I’ll happily post them within Germany. First come first served. I only have two. They’re probably still about 80% good, so they’ll most likely last another year or so before you can no longer charge them suffiiciently to make them useful.

73 Ed.

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I started on 80m SSB on this morning’s activation of Gun G/SP-013, and the PWR indication was at just 2 bars (SWR indication 0 bars, so antenna all fine after yesterday’s repair). Nonetheless, even at that I was quickly received and worked by chasers, and good reports (58, 57, 59 etc) incoming. The PWR indication crept up as usual to something that look more healthy after 10 minutes or so.

As it seems this is not really affecting my ability to be heard by chasers, and the fact that others have also noted similar behaviour of the 817s, I am not too worried about this.

The fact that I can sit directly under my antenna and operate on FT8 no problem, but suffer prohibitive feedback issues on PSK remains a mystery.

Issue 3. is solved. On the newest version of WSJT (2.1.2) the waterfall just needed to be fully dragged out to the right in order that the signals in the top half of of the band could be decoded!

Hi Tom,
Sorry if you’ve already answered this but are you only going by the FT817 power indicator or do you see this slow-rising output power through an external RF power meter as well?

73 Ed.

Yes, just from the 817 “meter”.

I realise this has its limitations, but I’ve always found the SWR indication on the 817 reliable enough for building/matching antennas etc.

My thought is purely to make sure yoy aren’t seeing a metering problem in the 817 rather than an actual reduced output and as you say you have no trouble making contacts even when the 817 power meter says you have reduced output, it could possibly be a component in the metering circuit only and in fact you are getting full power out all the time - just a thought. I “think” the metering circuit is just for HF - i.e. 6m and 2/70cm have a separate circuit.

If the problem is repeatable putting a power meter or SWR bridge between the rig and the antenna to check might be a good test to rule out that it’s not just a metering problem.

73 Ed.