Hamlib could be fairly easily extended to support reporting IF offset, but it would still only work well with a  downconverting radio.

Actually there might be something to be said for a small board with a buffer/mixer/Si570/small micro combo, putting the panadaptors IF some place other then the receivers IF has advantages even if the first IF is somewhere useful (Less chance of leakage back into the main RX).

I still don't think there is a market for it, but it could theoretically be done.

73's Dan.

Correct answer is indeed C, assuming base 2.
I cannot see a sane way to make it A.

Methinks a practice question is incorrect (It would not be the first time).

Regards, Dan.

Hi all,
I have just been playing with a notion and wanted to see what others thought:

I was playing with a speech band limiter in DSP (clipping in DSP is a pain in the bum, think polynominal approximations to erf()), and the thought occurred that the phasing method of SSB generation had some interesting possibilities if you replaced the sine and cosine oscillators with an analytic pair formed by taking the hilbert transform of the gain control signal.
In this way all the IMD could be caused to lie on the upper side of the voice signal where in octave terms it is going to be closer to the audio and where the worst of it can be filtered out by a post clipper LPF (followed inevitably by overshoot control measures). 

Some careful delay equalization will be needed to ensure everything arrives where it needs to be when it needs to be there, and this really needs to be applied only to components well above DC as the hilbert transform does not work at DC, still by early experiments in octave are allowing rather heavy limiting with far less apparent distortion then my simpler attempts!

Given that DSP cycles are cheaper then peak envelope power, there may be quite a lot to be said for pushing the audio processing to something rather smarter then much of what is typically seen in amateur service.
Even something like a 'phase rotator' (allpass filter with a non constant group delay) can increase average modulation by 3db on commercial AM sites, so it may do something useful for SSB as well.

Of course increasing the average transmit power will mean that the cooling and power supplies will have to man up a bit to cope with it but even so it has to be a smarter approach then just using a bigger amplifier.

Thoughts?

Sigh.

While I personally think that refusing to supply gear to someone who writes a bad review of a previous purchase is poor PR, that is their decision and is not really germane to the discussion of the usefulness or otherwise of anonymous reviews.
Neither in fact is the fact that on this occasion the comments on the radios very poor TX IMD are borne out by the measurements in the manufacturers own literature. 

Now personally I can and will judge a reviewers credibility on past history of talking sense, and on how often I find myself agreeing with their reviews of kit that I do own, none of which is possible with an anonymous review, but conversely an anonymous reviewer might happen to point something out which is of importance to me, so I see no real reason to ban them. Give them the weight they deserve when reading, sure, but sometimes there is something there that matters to my particular use case.

When considering a purchase you should actually read the reviews and apply appropriate weightings, going by a single number is a poor way to make a purchasing decision, I don't really understand the point of having single figure of merit like 4.5/5 for something as multifaceted and complex as a radio or piece of test gear, it does not (and cannot) tell you anything useful.

Some clown posting an anonymous 0/5 for a K3/FTdx5K or whatever your shiny toy is, really is not a big deal, just ignore it and move on (It might even be legitimate if the radio has a showstopper issue for that users particular use case).

It might be an annoyance if you were the manufacturers marketing slime, but watch me cry, I really don't care.

Regards, Dan.

Yours does Jazz? I didn't think the market was there to make it pay!

That Harris is still 200KW of heat, impressive for an air cooled sender.
I have seen air cooled dummy loads in that class, they call them load banks and use them to test medium to large electrical generators, never seen an RF load that big however.

Yea, narrow band class C stuff for FM can be wonderfully small and relatively light even in CCS grade, pity linear amps do not usually do so well. I do wonder why nobody is experimenting with full on polar loop transmitters for amateur service, the reduction in heatsinking should pay for the extra processing, and it is not like switchmode audio amps are anything very special these days...
 
I might have to play with it sometime.

Regards, Dan.

Might there be a fuse or internal power connector that has become dirty or loose.
It does not take much to drop a volt or so at a few tens of amps load current.

Stick a 'scope on the internal power bus somewhere suitable and check that it does not sag on load, if it does, find out why.....

Regards, Dan.

I was actually thinking more about the sort of 2/3/4U boxes you see doing maybe a couple of hundred watts on FM, pretty much all the non pony examples have a filter fitted on the front panel, and/or do the tunnel cooling thing.

The big long/medium wave senders are of course a whole other ballgame, but are seldom (directly) air cooled, .

You are saying that your transmitter only phones up to bitch that you are not paying her enough attention?

Regards, Dan.

Sigh, this sort of penny pinching is why I tend to roll my own!

Pretty much EVERY broadcast service rig I have ever see that is small enough to be air cooled has either used a fairly wide tunnel cooling arrangement (That pretty much does not need a filter), or has a dust filter fitted at the intake. Blowing unfiltered air directly over the boards is a recipe for fluff build up and poor reliability.
This goes double in a humid environment or one exposed to wide temperature changes as the fluff turns into a gungy mess that shorts things (Particularly a problem in AM class E stages, which run high voltages).

An air flow switch is good if you do the filter thing, as are some thermal switches in carefully chosen locations (which are a good plan anyway).

The filter screens are easy to wash out in the sink, usually done at the same time I hook the analyzer up and do the spurious and harmonic emissions checks, so every few months.

VSWR foldback will do in a pinch in place of over current shutdown, for all that I like both, especially if running multiple power stages with splitters and combiners where shutdown on excess current imbalance between the modules is a good and happy making thing. 

There is a reason my homebrew rig cost roughly a FTdx5000MP while offering less pretty lights and knobs (and being rather larger)!

Regards, Dan.

You can find the impedance with a signal generator, a 'scope and a selection of resistors.......

Hook the 50R output of the generator (set to square wave) across one end of the cable with the 'scope in parallel, then sub resistors at the far end until the reflection goes away, job done.

Having a reasonable length of cable helps as the frequencies involved become lower.

Regards, Dan.

I am no tube amp expert, but one thing that looking at the photos I would definitely be looking to replace is that 2 conductor power cable.
It may just be 240V land talking, but a non earthed case on a linear scares me.

Yes I know they shipped like that, and that the US tends generally to a MUCH more casual approach to safety ground connections, but still.

it would be embarrassing to be taken out by a simple live chassis fault while working on a valve linear, no?

Regards, Dan.

I (in common with most UK hams) have been somewhat insulated from this particular problem because our legal limit is 400W at the feedpoint, which means that an amp bigger then 500W is largely pointless (And the 1K amps that are out there tend to be run way below full power, no point in running a pair of 3-500s red or yellow for that output).

At that power level semiconductors are an entirely practical approach, and further with 400W being only 1 S unit over 100W, there is less incentive to bother.

Agreed that if you are designing a high power grounded grid amp, then spark gaps are cheap insurance (certainly compared to loosing a filament transformer), as are things like glitch resistors (Make sure they are rated for enough voltage), and series diode strings across the meters, and cathode bias arrangements.

There used to be a device sometimes seen at the tube base in old tellys that was a sort of combined spark gap and suppressor capacitor, quite useful but I have not seen a source for the things in quite some years.

Regards, Dan.

The HT mic will very likely be expecting a DC bias.

Unplug the mic and measure between the audio input and ground, if there is no standing voltage there, then the rig does not internally provide the required bias, so hook a 4K7 resistor between the +8V line and the audio input, inside the mic plug.

Chances are that the mic will then work correctly.

Regards, Dan.

I would note for anyone needing them that mouser do a broad range of these things.

Also putting one behind the RX BPF (where present)  will help as an addition to the one right at the aerial socket as the one behind the BPF will have the BPF slowing the risetime of the pulse as well as limiting the energy.

Doing this has two  advantages: the network at the connector then only has to protect the BPF from surge damage (generally the BPF is more robust then the first mixer!), and the mixer protection network is not exposed to the full bandwidth at the aerial socket so a network there will be less prone to generate IMD problems.

Another trick: Design your T/R switching with two relays so that when power is off neither the transmitter nor the receiver is connected to the aerial (and possibly so that the aerial connector is grounded, there are at least two schools of thought on this!).

None of this will protect you from a direct hit, but near misses that induce energy into the aerials are far more common.

Regards, Dan.

Apologies if this is teaching grandmother to suck eggs....

Is the mic expecting power and if so, does the rig provide it?
Some of the old mics were dynamic elements that do not need DC bias.

Just a thought.

Regards, Dan.

Yep, always worth going in there every year or so, especially if the blower does not have a dust filter fitted (in which case this needs cleaning periodically).
Blow out the dust, check for any swelling of electrolytic caps, have a look for anything discolored and generally make sure everything is tight (Including crimp blade connectors, very often badly crimped in some gear).

It usually takes about an hour to get a radio apart far enough to get the dust out and to do a quick check around (Then sometimes 4 hours of swearing to get the front panel aligned).

I also favor running the fan at low speed all the time unless  things get hot at which point the fan should be sped up. This way (particularly if the heatsink is massive) there is a good chance that at least in SSB mode the low speed fan will pull enough heat out while on receive that it avoids any need to  run at high speed while on transmit.

Finally, do allow the rig to blow down after a heavy transmit session, 5 minutes with the power left on so the fan can run will pull a lot of heat out of the heatsink and will avoid the heatsink slowly cooking everything in its vicinity.

Regards, Dan.