Transmitter third harmonic received?

[Geordie McBoyne]

Been working on an Ekco A725 radio. Except for repositioning the tuning scale pointer there is  nothing else special to report about the set. Checked the calibration of the three short wavebands and everything is OK. Well , OK for a domestic radio receiver. 
Tuning through SW1 heard a familiar radio station, Greatest Hits which is transmitted on 1152Khz (261 meters) from a nearby transmitter site.  The sound quality coming from the station on the short waveband is terribly distorted and the frequency checks out to be 3.456Mhz. So is the transmitter leaking the 3rd harmonic? Must find a radio that'll tune to 2.304Mhz to check out if the 2nd harmonic is present.

Geordie McBoyne.

[ppppenguin]

This is the occasion to fire up a spectrum analyser if you have one. Or perhaps a software defined radio (SDR) to have a look at what's coming down the aerial lead.

If it's exactly 3rd harmonic it's unlikely to be a spurious response in the receiver such as an image etc.

[peter scott]

My first thought was an attenuator on the front end but of course this will be a ferrite rod aerial so a bit messy. Does it still do it when rotated to the null?

Peter

[Murphyv310]

Hi.
I'm not familiar with the radio but it won't use the ferrite rod for short wave, it'll be an external aerial you need to connect.
I would have tried from here but I very much doubt it will be receivable, although after dark is possible. I can use my SdrPlay RSP1A via the loop or a Chinese portable that is exceptionally good, an XHDATA D808.

[peter scott]

Deleted.

[Geordie McBoyne]

The Ekco A725 has a ferrite rod aerial for the medium waveband and the three short wavebands are served by aerial and earth sockets. For short wave reception the receiver is connected to a length of wire of four meters.

Geordie McBoyne.

[peter scott]

Does it have a tuned RF stage? Can you see the signal running into clipping at the front end?

Peter

[Geordie McBoyne]

Hi Peter
No RF amplifier, the aerial tuning coils simply feed the signal grid of a Mullard ECH81. This valve is said to be the ultimate frequency changer and because of it's performance is so good an RF amplifier is not necessary in most receivers.
Tomorrow I'll try another short wave receiver and report back my findings.

Geordie McBoyne.

[Mike Watterson]

Actually with a strong signal out of band and the AGC going to maximum, the 3rd harmonic can be generated in the Hexode.

An RF preamp isn't just about gain. It can be a buffer for an extra tuned stage to reduce out of band intermodulation (and a signal can "modulate" itself). One of the reasons for the Triode-Hexode is actually to reduce LO leakage on the RF in pin, which is severe on heptodes, octodes and self oscillating mixer using triodes. Hence VHF valve amps using either a balanced input or an RF stage with almost no gain to stop LO radiation on the roof top aerials. US Broadcast only receivers, battery or mains, often used an RF amp before the heptode/octode purely to reduce re-radiation.

Any triode-Pentode, hexode-triode or heptode etc with a separate oscillator is superior to a combined mixer/oscillator using a single active device. The ECH81 (=ECH83) is very good, but not as good as a multiplier (7360, ring diodes, Gilbert Cell) with a separate oscillator. It's good because the triode part is a separate oscillator. This also removes FM of the LO by a strong signal, a failing of single device mixer/oscillators such as the heptode and triode/transistor mixer-oscillator.

I've an open mind as to if it's really a third harmonic. I'd put a low pass filter (1.8MHz approx) on the input and also compare a signal generator at that frequency and other on MW at higher level.

Or add a tuned SW circuit with a unity gain J-FET buffer (follower) to see if it's an artefact of the hexode. You CAN use any amplifier, hexode or not, to get a 3rd harmonic by the non-linear action of multiplying, which may be more likely at certain bias levels, so also disconnecting the DC AGC bias (if used on the ECH81 hexode) and applying 0 to -9V via 1 M Ohm might be interesting.

Oh, and I had a set where the Triode part (oscillator) of the UCH81 wasn't running due to a resistor gone high. It would work at the low end of MW and high end of LW on the generator, with tuning. I think simply leakage past the 470 KHz IF and it acting as a TRF! It was a very high level.

[peter scott]

I guess my thoughts were a bit silly given the likely signal sizes. I was just amazed that a broadcaster would be putting out blatant harmonics. Connecting a sig gen at 1152kHz to the aerial input would be interesting.

Peter