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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 12th Jan 2023, 3:56 pm   #141
Bazz4CQJ
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Default Re: Questions on 160/80 metre AM transmitters.

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Originally Posted by G6Tanuki View Post
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.
Yes, this the main reason I have been trying to find the circuit of the early Pye Ranger modulator as I suspect it had filtering and possibly compression. I'm not proposing it is anything special, or that it should be copied, simply that it might be interesting to see. My recollection is that the commercial RT sets had quite low levels of mod, nowhere near 100%.

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Old 12th Jan 2023, 4:20 pm   #142
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Default Re: Questions on 160/80 metre AM transmitters.

When I worked at Pye in Manchester I sort of remember likes of the W30AM, Cambridge etc got quite high levels of mod at around the 90%.

I might try a small pre-amp/clipper circuit before the mic socket at some point trying to keep the TX unit basic as the lines of the CODAR and others.

I did think about winding some extra turns on the autotransformer to provide a feedback winding and have it feedback to negatively bias a grid, or the is still the idea of the Neon and the LDR on the input! I did try playing with LEDS but found they needed quite a few mA's to drive the LDR down enough, perhaps I had the wrong LDR.

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Old 12th Jan 2023, 4:47 pm   #143
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Default Re: Questions on 160/80 metre AM transmitters.

Certainly the Pye AM15 Westminster and W20AM/FM Whitehall could do 100% modulation or better; problem was, Pye back then had two different microphone-inserts with different frequency-responses/outputs for AM and FM. Put a FM mic on an AM radio and it would sound 'thin'.

Sticking with the CODAR-style circuit then experimenting with outboard audio-processors sounds a sensible path; it certainly makes it a lot easier to change things round to see which gives the results you like.

There was a bit of experimentation with 'interesting' modulation schemes in the US in the 1950s: https://www.rfcafe.com/references/ra...ruary-1950.htm

and the attached PDF which shows the 'negative cycle loading' thing with a high-power diode on the secondary of the modulator transformer with a potentiometer to set the point at which the diode loads the secondary on negative voltage-excursions.
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File Type: pdf 5610027[1].pdf (283.0 KB, 33 views)
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Old 12th Jan 2023, 8:05 pm   #144
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Default Re: Questions on 160/80 metre AM transmitters.

I use this on my main AM 80metre Transmitter:-

https://www.s9plus.com/FAT%20MAX.html

Its a nice little kit that sits between the mic and the mic input socket on the transmitter. It helps to limit the bandwidth of the transmission and can be varied between 4Khz and 10Khz. It can keep the trasmission to the recommended 6Khz limit for AM on 60 metres ( 5.317 Mhz ), not that I go on 60 at the moment. It does have some compression built in, which can also be varied.

Cheers

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Old 12th Jan 2023, 11:08 pm   #145
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Default Re: Questions on 160/80 metre AM transmitters.

That looks interesting. Can you flick a switch to have it in or out, and if so, how do the signal reports compare? A long time ago, people would sometimes describe someone's modulation as "BBC quality", meant as a compliment, but actually not.

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Old 13th Jan 2023, 11:31 am   #146
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Default Re: Questions on 160/80 metre AM transmitters.

Hi Baz

I don't have switch on mine, to put it in or out, but it would be easy to fit one. The kit, by default, has the bandwidth set to 6Khz, probably with 60 metres in mind, but I've put a selector switch on mine and, with suitable extra capacitors, can switch between 4, 6, 8 and 10Khz. Also, I've put a potentiometer on the front panel, which varies the level of compression. The whole thing is more useful for controlling bandwidth , but it does provide compression, too much in fact if I turn the pot too far.

The sound of my transmission is probably more communications quality than BBC, in part due to the Pye tulip mic I mostly use. I sometimes use a Pye Westminster mic, which gives a fuller sound but the lead is shorter, so not a convenient

Cheers

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Old 13th Jan 2023, 11:44 am   #147
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Default Re: Questions on 160/80 metre AM transmitters.

Yes, I think having a bypass switch (perhaps a biased switch so you cannot accidently leave it bypassed) would be an interesting mod. I suspect that you could soon confirm that the processor was money well-spent.

The only working Pye RT I had was the AM Vanguard, and I still have the mike from that. That was definitely under-modulated and I added a pre-amp on the modulator. Ah, the whine of the inverter, the smell of charging car-batteries .

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Old 13th Jan 2023, 12:26 pm   #148
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Default Re: Questions on 160/80 metre AM transmitters.

Here's a little picture of my main 80 transmitter. The Fatmax box is to the right, sitting on top of a little set of preamp/tone controls I sometimes use.
Pye Tulip mic below
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Old 13th Jan 2023, 1:12 pm   #149
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Default Re: Questions on 160/80 metre AM transmitters.

What's the PA valve in that and what antenna do you have?

I have dreadful QTH re-antennas. The house is right in the centre of the plot, and the front garden has overhead cables for phone and electricity. I put up an inverted V for 80m with the apex suspended from a tree in the back garden, but it blew down and my tree-climbing days are over.

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Old 13th Jan 2023, 3:00 pm   #150
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Default Re: Questions on 160/80 metre AM transmitters.

It's two 6146 in parallel, but they only have 450v on the anodes, so hopefully should last a long time. The aerial is a half wave inverted V dipole, with the ends bent back a little to get in the space. The main support is in the centre - it's a 6metre fiberglass pole, mounted on the gable end of our bungalow at the back. The height at the centre is about 30 feet.

It's only good for 80, as a resonant aerial , but I did strap the co-ax ends together and tuned it against earth , with an AMU, for 160 some years ago.

Yes, it's very nervy climbing up a ladder when you get past a certain age. I managed it about 3 years ago. Not sure I'd want to try now.

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Old 13th Jan 2023, 3:16 pm   #151
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Default Re: Questions on 160/80 metre AM transmitters.

My inverted V used to load very nicely. That said, for reception, my magloop was perhaps the better antenna (and potentially, directional), so much so that I modified the TS530 I have so I could receive on the magloop and transmit on the inverted V.

I haven't used the 530 in years; were I to build an 80m AM rig, I could steal the pair of 6146's from the 530.

I guess the fibreglass pole is in sections? I could easily stick one at the back of the house.

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Old 13th Jan 2023, 10:29 pm   #152
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Default Re: Questions on 160/80 metre AM transmitters.

Yes I've heard that magloops can be very good for reception. The fibreglass pole is in one piece.

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Old 13th Jan 2023, 11:52 pm   #153
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Default Re: Questions on 160/80 metre AM transmitters.

Would a mag loop calculator work for a frame aerial?

It must be very inefficient?

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Old 14th Jan 2023, 3:11 am   #154
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Default Re: Questions on 160/80 metre AM transmitters.

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Would a mag loop calculator work for a frame aerial? It must be very inefficient?
Adrian

Not heard of a magloop calculator. The type of magloop I'm referring to is of the Wellbrook /Wellgood ilk. On 80m, the signal strengths on my Wellgood are a tad down than on my inverted V, but the background noise level (i.e. digital harsh) is about S8 on the V and virtually nothing on the magloop.

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Old 14th Jan 2023, 11:09 am   #155
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Default Re: Questions on 160/80 metre AM transmitters.

I must be getting confused with another thread I have been reading when someone pointed to an article in PW Nov 66 about a MW Loop/Frame aerial at around 40" sides. I always assumed that aerials were reciprocal, if good on RX, good on TX.

So wondering how to figure out if a low power, i.e. 5 watts output could transmit into a frame aerial. That aerial used 28 metres of wire and a 4 meter coupling loop, so to me I thought of it as a MW Mag loop antenna. Tuning would be very sharp, but wondering how it would work if at all.

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Old 14th Jan 2023, 1:25 pm   #156
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Default Re: Questions on 160/80 metre AM transmitters.

The whole subject of loop antennas can get one going around.... in loops!
The magloops are active receiving antennas with a small amplifier built on to them. In reading around about magloops, I have become aware that there is a great deal of current interest in using loop antennas for transmission.

I think the current interest is partly a result of many countries becoming more more restrictive about having very conspicuous ham radio aerials in residential areas. The transmitting loops need tuning capacitors to make them work, and even at modest power levels, the voltage across those caps is appreciable, and this has spawned a secondary hobby; build your own wide-spaced tuning caps.

Judging from what comes up on Google and YouTube, people seem to be having a lot of fun with transmitting loops, but it's not an subject I've looked deeply.

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Old 14th Jan 2023, 4:22 pm   #157
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Default Re: Questions on 160/80 metre AM transmitters.

Yes, *transmitting* loop antennas are rather different to the active-loops used for receive.

Compact transmitting loops need very low losses [both resistive in the loop itself, and dielectric in the capacitor] as you will typically have tens/hundreds of Amps circulating in the loop and quite a few Kilovolts across the capacitor, even with quite low transmit powers.

I have memory of seeing one such loop being developed for a client who shall remain nameless; the loop was made out of copper tube about 3 inches diameter, the capacitor across the ends was about the size of a bucket, made out of glazed ceramic, and with 'corona rings' to prevent flashover across the outside. To adjust the capacitor there was apparently a piston-and-diaphragm unit iside the ceramic bucket, and this was operated by hydraulic pressure from a nylon tube that ran coaxially inside the loop down to the bottom. A previous version using a stepper-motor to drive the capacitor had resulted in a small fire...

All this for a mere 500 Watts power-handling of two-tone FSK. And it could only be tuned over about 50KHz from its design frequency!

Here I've played with a 14MHz transmit loop in the attic. Flashover of the capacitor [an Eddystone VHF transmitting 'butterfly' type] on SSB peaks means I can't push it to more than 100 Watts or so.


Of course there's no need to use the same antenna for transmit and receive.... use a nice outdoor antenna for efficient launching of your RF, and a loop on receive for its superior directionality and noise rejection. Just make sure you have good RX/RX switching...
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Old 14th Jan 2023, 6:15 pm   #158
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Default Re: Questions on 160/80 metre AM transmitters.

There are few transmitting loops described on the net where people have used adjustable vacuum capacitors. These are more commonly available than might be expected; quite a few on ebay as we speak, though I've not looked at them in detail. They are not cheap, but not out of the ball park if someone is really keen.

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Old 18th Jan 2023, 12:37 pm   #159
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Default Re: Questions on 160/80 metre AM transmitters.

I did some testing of my little TX last night, I wish I had taken pictures of the display on my test set but unfortunately did not.

Using the Codar circuit for the multiplier section, I fitted a variable coil and 22pf trimmer cap so I could try things out on the 3.6 MHz area.

First I did some initial tests on 1.9MHz and I could get 6 watts into the test set and checking for spurious found these to be around 40dB less than carrier, Which I guess is just about acceptable.

When tuning for 3.6MHz output, I could get that just by tuning the PA Tuning cap and loading coil, but I then got spurious signals at 5.7 MHz and at 7.6 MHz. Switching the extra coil and cap in and having a play with the inductor I found I could null out the 5.7 and 7.6 MHz signals, but found I would have to retune the coil to re null, if I changed the VFO by more than a few kHz. When properly nulled these were 40 dB below carrier.

In all instances though I found that 1.9 MHz was at best 25dB below carrier, so the fundamental was still getting out as well as 3.6 MHz. This backs up Paula's (frsimen) comment in post 24

Quote:
Originally Posted by frsimen View Post
It should be noted that, while better than nothing, there is still a significant amount of the 1.75-1.9MHz present on the output of the AT5 when used on 80m.
Paula
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?

A FAT5 audio processor kit has just arrived.

Adrian
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Old 18th Jan 2023, 4:55 pm   #160
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Default Re: Questions on 160/80 metre AM transmitters.

My preference [and what I am using in my homebrew 5763 transmitter] is to use a parallel-tuned-circuit in the PA anode circuit, with a link coupling winding around the 'cold' end as the output.

This then feeds a symmetrical band-specific low-pass-filter [3 toroids, 4 capacitors] designed for 50 Ohms source and terminating impedance, and then via coax into the antenna-matching unit.

I have always felt that the 'classic' pi-tank-with-two-variable-capacitors is a bit of a compromise, intended to allow the PA to be conveniently matched into antennas that are reactive rather than purely resistive, but this means that you rarely arrive at the optimum values of L and C to perform the low-pass-filtering function. Separating the impedance-transformation and the low-pass-filter functions allows them to be separately optimised.

My approach also means you only have one variable capacitor.
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