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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 18th Jan 2023, 6:15 pm   #161
Cruisin Marine
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Default Re: Questions on 160/80 metre AM transmitters.

Now I find this level of -25 dBc to be too high and I am considering just using the box as a Top band only TX as -40 dBc is just about acceptable.

Before I go further I am wondering if I have enough space in my layout to fit either a tuned driver anode or tuned PA grid L and C Tuned PA grid may be easier, but think the impedance may be to low when compared to the 22K resistor that is there.

I would appreciate others thought on the spurious levels and what you would consider acceptable?


I would consider -40 dB or below OK, that is what a Yaesu FT901 promises for example.
I never owned a Codar AT5, but many many decades back I recall hearing one way out of band, I can't remember what freq. (too long ago), but it was quite shocking.
That driver to PA design is really a bodge.

Tuned Anode to grid is the best way to go with either a tapped coil or switched C's. Tapped coil would be best.

It shouldn't take up any more space than an RFC, but you may be using mini RFC's.
You'll crack it, I am certain.
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Old 18th Jan 2023, 8:29 pm   #162
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Default Re: Questions on 160/80 metre AM transmitters.

You could consider a high pass filter on the output to remove the fundamental when on 80m. You would need to switch it out of circuit for 160m operation, of course.

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Old 18th Jan 2023, 8:36 pm   #163
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Default Re: Questions on 160/80 metre AM transmitters.

Long post again sorry.

I decided to go through some of the tests again as I had slept the night and could have been out on my numbers, and I also wanted to try a PA grid tuned circuit as well.

I also took note of the PA grid voltages to determine how much drive was getting to the PA, there is a 22K resistor from ground, at the top end of the resistor a 0.01uF cap to ground and then a 1.5 mH choke coil to grid, that way at the junction of the choke coil and 22K resistor I can take voltage readings without the DVM being upset by the RF.

This time I took a few pictures as I went along.

The coil is a core tuned pot core meant for TV IF's and tuned from 100uH up to 500uH. I used a fixed capacitor this time 15pF ceramic (750 Volt). One less thing to twiddle then the 22 pF variable. This are in series between the driver anode and ground.

Click image for larger version

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This is tuned for 1.9 MHz as it is, no additional coil/cap.
PA grid -40 Volts

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This is tuned for 3.8 MHz as it is no additional coil/cap.
PA grid -43 Volts

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This is tuned for 3.8 MHZ but with the 3rd harmonic of the fundamental (5.7MHz) notched down with the coil/cap.
PA grid -26 Volts

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This is tuned for 3.8 MHZ but with the 1.9 MHz fundamental notched but not out altogether with the coil/cap.
PA grid -8.4 Volts


I also tried the coil and cap in parallel across the grid of the PA valve and whilst I could tune these to 3.6 MHz the level of drive was very low, at this frequency the coil was only around 100uH and it's reactance would be circa 2400 Ohms which would be very low compared to the normal 22K Ohm grid load, perhaps I did that a bit wrong?

I tried a tuned driver anode side as well without a lot of success using the small cored coil, it would give lots more drive to the PA grid as in circa -100 Volts, but the end effect on spurii was not a lot different, it also would mean I have to tune the driver anode as well as the PA anode tank and loading.

So still thinking about it, I will leave it as a 160m TX only and do some switching to give CW or AM options as I would consider it clean enough just for Top Band use. For 80 and above I could always switch on the FT902. It's just that it is still 100 watts on Top band which is a bit naughty.

G6Tanuki, general question about your system, yes you have only one PA tuning point, but you must still have 3 tuning points in your ATU? But I have a thought The idea was to build something simple low power and in one box that I could load up into a long wire direct from the ant socket, perhaps a daft idea granted.
But here is my question, would there be any benefit to keeping the low pass filter at the impedance of the PA valve, circa 2900 Ohms and then having a PI matching circuit before the antenna connection, smaller wire dia, less current in the inductances?

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Just wondering.

Adrian
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Old 18th Jan 2023, 8:50 pm   #164
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Default Re: Questions on 160/80 metre AM transmitters.

Quote:
Originally Posted by frsimen View Post
You could consider a high pass filter on the output to remove the fundamental when on 80m. You would need to switch it out of circuit for 160m operation, of course.

Paula
Yes that is always possible, but not a simple solution now I have the box built, one other possibility I could try, is to drive it with a 3.6 MHz VFO for 80 metres, that way there would be no 1.9 MHz input and the second harmonic would be at 7.6 MHz which would be filtered out better with the PI network. I could drive it with DDS freq generator I have somewhere in my junk, but a bit of a cheat. I have a sort of idea brewing for a simple mixing type TX, but that will be very much further down the line.

Cheers

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Old 18th Jan 2023, 9:15 pm   #165
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Default Re: Questions on 160/80 metre AM transmitters.

Another very simple option is to add an extra capacitor in parallel with the Pi tank Inductor, to make the tank L then a parallel trap to stop the unwanted!
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Last edited by Cruisin Marine; 18th Jan 2023 at 9:39 pm.
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Old 18th Jan 2023, 9:36 pm   #166
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Default Re: Questions on 160/80 metre AM transmitters.

That is a thought, something like 16 or 17pF across the Tank coil (44uH) may tune out some of the 5.7 MHz, I could see how effective that work over the 5.25 to 6 MHz range, the 1.9MHz signal could not be treated that way when on 40, even if I include the 1.9 MHz notch on the driver anode I would still get around -30 from that.

I think I was expecting too much from such a simple circuit?

If it was going up to a tuned antenna I could expect a few less dB's for the spurii being out of the antennas band width, but it is not something I could measure and thinking along the lines of a long wire difficult to predict I guess.

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Old 19th Jan 2023, 5:44 pm   #167
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Default Re: Questions on 160/80 metre AM transmitters.

the 1.9MHz signal could not be treated that way when on 40, even if I include the 1.9 MHz notch on the driver anode I would still get around -30 from that.

Have you tried it?
If it only achieves -30, then also add a series LC resonant at 1.9 MHz across the antenna socket when operating on 80m, that would surely reduce it to much more than -40. If you reduce that leak through, your other problems may go too
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Last edited by Cruisin Marine; 19th Jan 2023 at 6:03 pm.
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Old 19th Jan 2023, 7:02 pm   #168
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Default Re: Questions on 160/80 metre AM transmitters.

No not tried it yet, noticed I put 40 when I should have put 80.

Today has been building up a FAT5 speech processor kit and fitting a slow motion drive to the VFO. After the kit my eyes are a bit tired having to look at things with magnifiers, in the end I just got the DVM out to determine values of components. No wonder I like valve gear.

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Old 20th Jan 2023, 1:09 pm   #169
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Default Re: Questions on 160/80 metre AM transmitters.

The problem with trying to parallel-tune the pi-tank inductor to act as a rejector on the 2nd harmonic is that any added parallel-capacitance will be varied when the two pi-tank capacitors are varied [since these effectively are in series across the inductor too].

I would do all you can to make sure the drive to the PA is as spectrally-clean as possible, on the basis that with the legal-maximum 10 Watts DC input to the final, you want all of those ten watts to be used to amplify the wanted signal rather than some of it being wasted on ampllifying unwanted harmonics!

In the days when I was dealing with RF-stuff, we didn't look at harmonics/spurii as being "X dB down", we looked at the energy of the harmonics; the spec we aimed to meet would typically say "in the range 1 to 40MHz, no single harmonic or spurious emission will exceed 1.5 Milliwatts into 50 Ohms, and the aggregated energy of all such emissions will not exceed 10 Milliwatts".

Of course this lets low-power gear off with less-strict filtering/linearity needs than high-power stuff, but that's only fair because the receiver your harmonics/spurii are interfering with doesn't interest itself with the cause, it only experiences the effects!

Some amateur-band stuff in times-past was really not that good; I remember annoying the late G3HRN by telling him that I heard him calling CQ on 20 Metres when he was actually calling CQ-80. Yes, I was hearing a harmonic, and he was a good few miles away.
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Old 20th Jan 2023, 2:32 pm   #170
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Default Re: Questions on 160/80 metre AM transmitters.

I can remember back in my days as a field installer, we were installing a North West regional system going from the Cheshire Plains up to Carlisle, Storno 19 inch base stations, there were meant to be spectrally clean to -90dBc.

The customer rep armed with a spectrum analyser parked out side a site and reported to me they were not meeting the figure, I asked how he was measuring it, 'with a whip on the analyzer'. An hour later I was on site with a big notch filter to remove the fundamental from the input, harmonics were then better than spec. I do think you are correct in that to get a clean signal I have to have a clean drive to the PA.

I will look at having the VFO run at 80m switchable if possible, or just leave it as a Top band set.
I can have a play with it as I still have to do NET, STBY and TX switching at some point on it, I am looking for a small ish rotary that has a widish wiper to give 3 active positions. Think I will head to Lincoln in a week's time to the rally.

Other then that, it can still make a good pantry TX just off the bulb and keeps the useable RF within the house.

Adrian
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Old 20th Jan 2023, 3:10 pm   #171
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Default Re: Questions on 160/80 metre AM transmitters.

Commercial spec is 0.25uW -36dBm up to 1GHz needing 76dBc attenuation for a 10W TX.

The Storno would probably have been 25W or 45dBm, 81dBc needed and the measured 90dBc gave some margin.
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Old 20th Jan 2023, 4:15 pm   #172
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Default Re: Questions on 160/80 metre AM transmitters.

I don't think anyone has come to grief over the harmonic output of an AT5. It's unlikely to interfere with significant services, although it probably won't meet modern harmonic suppression requirements, but for amateur radio those seem fairly vague anyway.

From Ofcom - UK AMATEUR RADIO LICENCE
Section 2
Terms, conditions and limitations:

The Licensee shall ensure that:
(a) the emitted frequency of the apparatus comprised in the Radio Equipment is as
stable and as free from Unwanted Emissions as the state of technical development
for amateur radio apparatus reasonably permits; and
(b) whatever class of emission is in use, the bandwidth occupied by the emission is
such that not more than 1% of the mean power of the transmission falls outside the
nominal modulated carrier bandwidth.

Unless I'm mistaken, that means the surprisingly low harmonic attenuation of only 20dB for amateur equipment is sufficient, although I'd want anything I built to have harmonic and spurious suppression to be at least 30 dB!

See here: https://www.ofcom.org.uk/__data/asse...teur-terms.pdf
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Old 20th Jan 2023, 4:53 pm   #173
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Default Re: Questions on 160/80 metre AM transmitters.

Harmonic levels never seemed to be a major topic of discussion in the designs of the Venerable F.G.Rayer .

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Old 20th Jan 2023, 5:12 pm   #174
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Default Re: Questions on 160/80 metre AM transmitters.

It should be worth noting that a 0.1 dB 3 pole Chebyshev LPF/PI tank will give no more than about 20 dB rejection of the 2nd Harmonic.
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Old 20th Jan 2023, 5:50 pm   #175
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Default Re: Questions on 160/80 metre AM transmitters.

Damn, missed the editing time limit point- but a 3 pole pole Chebyshev with 0.5 dB ripple will give 24 dB at 2f, and a 1 dB ripple type will give about 26 dB rejection at 2f.
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Old 20th Jan 2023, 5:57 pm   #176
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Default Re: Questions on 160/80 metre AM transmitters.

Hello Jon, I think the base stations were 40 to 50 watts output although this is going back years now, A mid band system that was being installed for the then North West Water.

Ian, thanks for the link, I would be more that happy to get 40dB down personally, that is what I appear to get on 1.9MHz when tuned up. It is the fundamental getting through that is my main concern when on 80m To do any Morse code I would be down near the bottom 1.75MHz which takes it out of the Amateur band and close to some coast guard stations. I am not sure how much would get out there probably very little I appreciate. 30 dB down on 6 watts output being 1/1000 at 6mW 40 dB being 200uW. is probably less than the Hash around here.

I just want to aim for the best or restrict it's use.

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Old 20th Jan 2023, 7:14 pm   #177
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Default Re: Questions on 160/80 metre AM transmitters.

Quote:
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[...] It is the fundamental getting through that is my main concern when on 80m To do any Morse code I would be down near the bottom 1.75MHz which takes it out of the Amateur band and close to some coast guard stations.[...]
Yes, the fundamental 1.8 MHz getting through on 3.6 MHz isn't attenuated by any low-pass filtering. But the acceptor circuit consisting of L3 and C9 makes it just about tolerable, especially if C9 is changed for a trimer and the fundamental is minimised for the bit of 80m used most often.

The AT5 is crude by today's standards, but even then it was a trade off between performance and complexity. Back in the day you'd have to be close (and unlucky!) to be a problem for any coast station.

Nevertheless, quite innovative and compact transmitter, borrowing some of the best ideas from the previous decade or so, with adding Codar adding some of their own.
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Old 20th Jan 2023, 9:34 pm   #178
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Default Re: Questions on 160/80 metre AM transmitters.

Having reflected upon all the inherent problems with the original Codar design, I can't see this nut being cracked properly without just restricting the design to just top band, or having a dual band/switched VFO.

It could of course be achieved with the aid of digital chips in the form of PLL and/ or dividers etc.
But that would take something away from the original concept.
Just my three penneth worth.
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Old 20th Jan 2023, 11:03 pm   #179
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Default Re: Questions on 160/80 metre AM transmitters.

I think the CODAR design was very novel and quite simple for the time, the fact it was copied by many is testament to the fact it basically worked. I do believe that spurii was not really an issue back then, or at least not really considered in any great depth.

I would need to build a separate VFO on a board to see if I could switch between 160m and 80m and be stable before any attempt to retro fit. One thing I could do is drive it initially with the DDS which I have now found, first I need to see how clean/jitter free that is, then see if it is a cleaner output.

An alternative is to add fixed crystal positions and go for additional 3.615 MHz and a CW spot on 80 as well as the VFO for 160.

Going back to something much earlier in the thread about the modulation, I realised I made a boo boo when I wound the transformer, I wound it symmetrically, i.e. same turns both sides of the centre. This is possibly why I can only reach 75 to 80% mod, I tried the dropping resistor and cap circuit to make up for the fact the mod valve has it's Cathode at 13 to 14 Volts above ground, it gave me a few extra percent, but the original transformer was asymmetrical being originally 110 Volts on one side and 130 Volts on the other, so I should have done one half with more turns. I may try this out to see if it gives me more modulation. The mod valve is drawing between 46 and 50 mA, the PA I am limiting this to 40 mA max and the residual DC level does not seem to be causing any issues!

I am enjoying the build and the whole process, it is still my main way of learning as opposed to just reading texts. I have to have something to relate everything to and try things out.

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Old 1st Mar 2023, 6:46 pm   #180
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Default Re: Questions on 160/80 metre AM transmitters.

For anyone thinking of building a rig similar to the AT5, here's some details of the neat little rig I built back in 1970 that was similar in size to the AT5.

It only had 4 valves, as I used an ECF80 as the VFO/Buffer. I chose a 5763 as the PA (really an RF version of the 6BW6).

The modulator used the same lineup - a 12AX7 into a 6BW6. However, I didn't use a centre-tapped choke, just a choke for the modulation. (you just connect the 6BW6 anode to the DC side of the PA RF choke, and the HT to the other side).

I just used a small mains transformer primary as the Choke, and the audio quality was actually better than from an AT5 ! (the transformer had a 6V secondary, so I connected that to a front-panel bulb, to indicate the modulation).

For 80m there was no switching - I just doubled in the PA - didn't even bother to tap the PA coil !

Oh and I managed to make the VFO very drify-free, just by selecting temperature compensating capacitors by trial and error.

Hope this may be useful to people.

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