Air cored coils

[mpegjohn]

Hi,
In my SW superhet, that tunes from 3MHz to 30MHz, I am looking at using 22mm poly tube as the formers for the RF amp and the tuned grid oscillator.
The 1st IF is 1.493MHz.

I aim to have 4 ranges.

To do the range switching, I am thinking of using relays. Can anyone comment on what would be the best type.

Can someone help on how to minimize interaction between unused coils? I had thought of using pcb copper clad to make a box around a coil and its trimmer/ padder, I am concerned about the size of all this.

Can anyone advise.

Thank you.
John

[Silicon]

Many radios solve this problem by having contacts on the wavechange switch that short out the unused coils.

[Wendymott]

Hi John. As Silicon says, Eddystone in particular use conventional switch wafers, and Shorting wafers to minimise adjacent effects. My EC10 conversion shows this clearly.
See attached schematic.

[Herald1360]

For range switching relays, DIL reed types are good for HF signal switching. Mercury wetted ones if you're really serious.....

[David G4EBT]

Don't know if it's of any help, but the G3TSO multi-band transceiver which featured in Radcom in the late 1980s, of which many hundreds were built, (using the long since defunct SL6xxx series of Plessey ICs), used diode band-switching. Below is a clip of the relevant part of the circuit, from the 1994 Radio Communications Handbook. I built all the modules, then lost interest in amateur radio and sold them at the G-QRP Convention, so I can't comment on the band-switching, which used both 1N914 and 1N4001 diodes.

(I recall that some years earlier, a solid state rig by the late Lorin Knight, G2DXK, used little reed relays for switching crystal filters in and out).

Might give some ideas.

Good luck with your SW superhet John.

[mpegjohn]

Thank you all,
The receiver is a valve set, so not sure how diodes would work?

[mpegjohn]

If I do use reed relays, would it be ok to put all related coils in series, and then just short out the ones not in use. I am a little worried what happens at 30MHz with all this hanging around!
Can I put all coild side by side, if the not in use cois are bypassed?

Thaks,
John

[TrevorG3VLF]

TO5 cased relays are available which, being small, will have low inductance. I do not know whether these are sealed with an inert atmosphere.

I read somewhere that gold flashed silver contacts should never be run above about 1V if they are to be used to switch a signal path due to the destruction of the gold layer.

[G6Tanuki]

I have used the TO5-cased SPCO relays [made by a US company called Teledyne] for low-pass-filter switching in a HF exciter unit - the "changeover" bit being used to short the in/out terminals of the filters to earth when they weren't selected, to prevent the risk of spurious 'suck-out' coupling/detuning effects on adjacent filters in the block.

[mpegjohn]

Trouble is that if I go for relays, I will need 2 per coil, with 5 ranges, 1 LO coil 2 RF coils I will need 30 relays.

The primaries of the RF transformers, and the tickler coil of the LOI, I plan to just use a sp relay and leave them open.
The secondaries of the RF and LO, I will use a SPCO and short them out when unused, and disconnect them from the rest of the circuit.

Teledyne relays are expensive.

[Tyso_Bl]

Can you show us what you're trying to do? Theremay be a way to do more with less, diode switching would be a good way to go.

[mpegjohn]

Ok I will try to draw a schematic.

[dominicbeesley]

Another option for the change over would be a push button arrangement - its a long time ago but I remember making something using 4 record/playback switched I'd harvested from old cassette recorders. The good thing with those is there are a lot of contacts to choose from.

I can't remember where I got the switchover mechanism but it was probably an old 4 button car radio.

D

[mpegjohn]

Here is a drawing I have done of the switching arrangements I think I need for my osc, rf amp in and out.
I have shown only one range.

Hope it helps to visualize what I think I need.

Thanks John

[Radio Wrangler]

The Teledyne TO5 relays are very very expensive, their reliability is OK, but not wonderful, I found.

Omron, SDS Relais, and several other firms do dual-in-line packaged relays with a variety of contact styles and materials. I found them to be capable of handling larger signals AND were more reliable. Many many more have been used than the TO5 jobs, so you're more likely to find them.

David

[G6Tanuki]

Originally Posted by Radio Wrangler

The Teledyne TO5 relays are very very expensive, their reliability is OK, but not wonderful, I found.

I used them in my design because they were already being used elsewhere on other parts of the project and so had an existing stock-number. On a "Cost-plus" contract you don't worry about how expensive the parts are!

One worthwhile thing to do with almost any relays being used in very-low-level-signal applications like antenna-coil-selection is to arrange for a milliamp or two of DC to also be fed through the switching contacts. This was my standard [though non-EMER] fix for 'periodic deafness' of the old C11/R210 radios caused by oxide-film buildup on the send/receive relay.

[mpegjohn]

Is there a way to use diodes?

[mpegjohn]

How about placing a diode switch across the secondaries and makin the spdt switch spst, which would lower the cost.
Would a diode switch lower the Q of the tuned circuit?

How about this as a rough design

[G6Tanuki]

It can be done with P-i-N diodes - this has been the standard way for 'solid-state switching' in mobile two-way radios for a few decades.

You need to make sure there's a good forward-bias current flowing through the 'on' diodes and plenty of reverse-voltage on the 'off' diodes - get this wrong and an inadequately-on-biased or inadequately cut-off diode will work as a mixer and generate traumatic levels of intermodulation-products/sproggies.

I generally don't like diodes in signal-paths.