Questions on 160/80 metre AM transmitters.

[Bazz4CQJ]

In place of an auto transformer, is there any chance that the primary of one of the 100V line matching transformers might work?
The kind of thing I'm thinking of is like this http://markhindes.easywebstore.co.uk/D29A100_AG31S.aspx.
The same guy has another transformer called a TS27 http://markhindes.easywebstore.co.uk/TS27_A9GZ4.aspxand there's more data around for that, see attached.

B

[G3VKM_Roger]

Originally Posted by Aub

Yes indeed. Quite clever.

Aub

I have the article by Dave Gordon-Smith/G3UUR that discusses the AT5 and also alternative mod transformers. This was published in the American "Electric Radio" magazine, so it is copyrighted and I won't post it here, but contact me by P/M if interested and I will find the relevant sections.

73

Roger/G3VKM

[John M0GLN]

There's also this one,

http://ekladata.com/ddX2c-NG42DyDMPd..._cq_jun_56.pdf

[Bazz4CQJ]

Earlier in the thread there was some discussion of VFO stability, particularly wrt temperature.

Am I right in thinking that a VFO coil with a ferrite core will have a larger tempo (positive) than a coil with no core (air)? The clear disadvantage to not using a core is that the coil in then not "trimmable". I guess also the a tuning slug is slow to reach a steady temperature even when the air around it has warmed but then stabilised.

So, I'm tempted to think that is a that not using an adjustable core is a good trade-off, or is that me being influenced by 50 years of HRO ownership

B

[Aub]

For best stability, I always use a ceramic former with no adjustment.

Cheers

Aub

[Bazz4CQJ]

Originally Posted by Aub

For best stability, I always use a ceramic former with no adjustment.

Cheers Aub

Yes - as an alternative to ceramic, what would you think of polystyrene, again, an HRO standard feature? Also, I have some polystyrene "pill bottles" of 1" diameter and they even have ribs running along the long axis.

Still thinking about your suggestion of running a VFO at half the op frequency. Needs a few more turns on the coil .

B

[Aub]

They should be ok, if not as good. I appreciate it's harder to come by ceramic formers. My first Topband rig, as pictured in this thread, actually uses a coil with a ferrite adjustable core, but sealed afterwards. It only ran at around 900Khz though, so needed the extra inductance that the ferrite core gave. When I used to work CW on it, as far as I recall I never had any complaints of drift. I guess it would have depended on the bandwidth of the other station's receiver.

Cheers

Aub

[G6Tanuki]

Ferrites can be designed to have positive, negative, or pretty-close-to-zero temperature coefficients - so don't dismiss them out-of-hand. In the past I built various direct-conversion receivers using ferrite toroids for the VFO and the drift was perfectly acceptable for listening to SSB/CW stations on 14MHz [SSB/CW have at least an order-of-magnitude greater sensitivity-to-drift than AM].

For me, the big thing is to keep the transmit VFO running all the time to prevent the thermal-cycling effects which come with stopping the transmit VFO when you're on receive. OK, you say, but won't leakage from the transmit VFO get back into the receiver and cause heterodynes/desense? Yes - but adding a diode switch [semiconductor or something like an EA50 according to your personal orientation] and a capacitor can happily shift the VFO a safe couple of hundred KHz away when you're on receive.

Alternatively, run the VFO at 2x the intended frequency and use a divide-by-two circuit. You can leave it running all the time then, and the divide-by-two will divide the drift by two too.

And doing other things - like mounting the VFO tuned-circuit in a nice solid thermally-stable diecast box - can probably do more to promote stability/long-term-resettability than fretting about coil characteristics. Short-term drift has rarely been an issue for me with VFOs below 5MHz; the more-annoying thing for me is long-term drift which compromises resettability and makes a nonsense of carefully-constructed calibration charts. Cooking a VFO in the oven [yes, heating it to 150C] several times over a period of days can help stress-relieve the construction and promote long-term resettability; I've got one here running on 4.7MHz [EF91 electron-coupled Colpitts, ECF82 cathode-follower/buffer] that settles to within 100Hz within ten minutes from switch-on from cold.

[Bazz4CQJ]

Ah, I didn't know about the alternative tempcos of ferrites; maybe they were all positive back in the 2m era when I last built a serious VFO, but using ceramics caps of various negative tempcos. The 11MHz VFO I put in a Vanguard receiver , and multiplied by 12, worked reasonably well, but the Rx was pretty deaf.

B

[G6Tanuki]

Yes, ferrites have evolved a lot since the early-sixties and the days of the EICO 753 [nicknamed the Seven-Drifty-Three or the Drifty-Fifty on account of its horrible VFO drift courtesy of inappropriately-specified ferrite cores in the VFO].

http://onetuberadio.com/2016/03/15/e...e-review-1966/

https://www.eham.net/reviews/view-product?id=2223

[Radio Wrangler]

Wouldn't think of using a ferrite toroid for VFOs etc. A gapped potcore maybe, but for a toroid as a filter or tuned inductor, I'd use an iron powder core, not a ferrite. A lot of Micrometals inc cores are on the market usually referred to as "Amidon" cores in amateur radio circles.

David

[Cruisin Marine]

Saw this some time back, nice simple idea https://www.qsl.net/py4bl/vfoesta.htm

[G6Tanuki]

Yes I've used Klaas's "Huff&Puff" stabiliser rather successfully in the past; it's a bit counter-intuitive to tune a receiver with such a stabiliser unless the 'stabilisation interval' is quite small. You tune the VFO to the required signal and then when you stop, it 'creeps' high or low by maybe 50Hz until it settles on the nearest stabilisation-point. You can't tell whether this is HF or LF of where you stop tuning!

But once it settles on a stabilisation-point, it stays there!

[Bazz4CQJ]

Mmmm, perhaps I should come clean and tell you that I have bought a DC70 valve. Used on MoD UHF walkie talkies, it has a quick-heat filament (1.2V, 200mA) which I suspect will give adequate emission well below those numbers. We are talking about ~1/10th the power used by an E valve heater. A D-valve VFO has been at the back of my mind for decades .

B

[AdrianH]

I wound an autotransformer for the mod stage taking as per the CODAR and it seems to work well, 80% mod is doable. The bandwidth is a bit wider then expected so I think some extra caps need to be added to bring any transmit bandwidth down a bit.

Update in the home mad section.

Adrian

[Cruisin Marine]

70-80% is about what is to be expected, it can be increased with an extra R-C in the modulated line. This is shown on p265 of the 1967 ARRL handbook.

It is also shown in this download on p243 of the '68 book https://ia903100.us.archive.org/23/i...8_handbook.pdf

[Bazz4CQJ]

I think I'm right in saying that you need quite an increase in power over a 10W Tx to significantly improve its range. Does anyone know how the modulation level affects range, e.g. what effect would increasing the mod from 70 to 80 to 90? be on range?

B

[AdrianH]

Thanks for that.

If I am reading it correctly and trying to apply this to the CODAR seesaw arrangement of the autotransformer, I would need to see at what point the mod anode drops to without distorting and if this is, say 25 Volts then I would need to drop 25 Volts across a resistor on the opposite side of the autotransformer, then C would have to be suitably large so as to bypass at the normal audio range.

So if it was 25 Volts at 40 mA R would be 625 Ohms and to pass a 100Hz audio signal without loss C would need to be just over 25uF which is a reactance of around 62.5 Ohm.

Interesting, it may not be worth trying for near 100% modulation, but an extra few percent to 90 may be worth a try.

Cheers

Adrian

ps I wonder if it would need to be a non polarised cap, it should not but I could see a lot of stress on a standard electrolytic.

[Cruisin Marine]

ps I wonder if it would need to be a non polarised cap, it should not but I could see a lot of stress on a standard electrolytic.

I just see it as a coupling capacitor and if the DC conditions are always more positive at the feed end than the load end then a non-polarised would not be needed, unless I am missing something?
Knowing the AC handling capability of the cap would help.
Having said that, a non polarised cap would put my mind at rest much more.
I agree 90% mod. would be a nice figure to achieve.
The other factor to note is that you may have to increase the HT to achieve the power output after introducing that R-C cct. That all depends on whether you want to keep it at it what you originally intended of course. It all is down to how much headroom you have to play with really.

[G6Tanuki]

Given the low RF power, making sure you achieve full modulation is all the more essential for optimising talk-power [hence, I suspect, why one of the designs previously referred-to uses a push-pull pair of 6BW6 as the modulator]. The big problem being to avoid swinging the anode of th PA down to a point where it goes into cutoff thus causing the tank-coil to 'ring' a bit like a LOPT on flyback and generate splatter.

Over the years there have been various tricks used - one being 'negative cycle loading' of the modulation transformer, where a diode is used to connect a resistor of appropriate value across the secondary of the modulation-transformer when the voltage is swinging too-negative. Of course this doesn't affect the positive drive so you can - theoretically - get over 100% modulation without distortion!

Another trick is asymmetrically clipping the drive waveform to the modulator, so as to clip the negative-going peaks but leave the positive-going ones unmolested. Again, you can get greater-than-100% modulation, but you need to take care that the resultabt shift of the DC working-point of the modulator and phase-splitter/driver stages does not cause issues [attention to time-constants!] and also make sure you get the resultant phasing correct or you won't be pushing the positive ide of the waveform.

Also, aggressive clipping of the audio: in practice the intelligibility of speech is primarily based on the frequency component not the amplitude component, so you can apply at least 10dB clipping - provided you follow it with some decent low-pass-filters to take out the harmonics that the clipping will generate.

A sine-wave reduced to a series of positive and negative trapezia! Nearly a square-wave - and we all know how that includes harmonics-to-infinity.

Plenty of audio-gain ahead of a diode clipper [a use for one of those little EB91 double-diodes that seem to breed in the dark corners of my office], along with consistent audio-drive from the microphone [I used to use one of the sports-commentator-type that has a metal lip-bar to maintain the optimum distance from your mouth] and you can push the modulation envelope consistently hard for maximum communications-efficiency.

[Old-style carbon microphones are good too, they always seem to introduce their own limiting-effect!]