Future Project Ideas

[NZ0I]

As the 80m ARDF loop antenna design has evolved, two thoughts have crystallized regarding follow-on projects:

1. Might it make sense to add a simple 80m CW receiver to the antenna PCB, creating a stand-alone 80m-only receiver? 

2. Might it make sense to add a simple converter circuit to the antenna PCB, creating an 80m adapter for use with VHF scanners and handhelds?

The second idea I am not so sure about. If it could be made to work, then it would provide a simple inexpensive tool allowing hams to do 80m ARDF with their 2m handy-talkies. It seems that it might be made to work quite well with VHF handhelds capable of all mode (or at least SSB/CW) reception. But for FM-only handhelds they might not get anything more than a carrier strength indication on their s-meters.

But what if the converter circuit applied some FM modulation to the signal used to mix with the 3.5 MHz received signal? It would seem that the product signal would also be FM modulated, allowing FM receivers to provide an audio tone to the listener. So the question then is, is there a straightforward (inexpensive and fairly simple) way to generate a reasonably-clean 140-MHz FM-modulated signal for mixing with the received 80m signal?

Anyone should feel free to comment if they have thoughts on this.

[NZ0I]

A few more thoughts on the subject:

The FM modulation might be accomplished as simply as using a VCO to generate a frequency to be modulated. The modulating signal could be generated using a 555 timer circuit to generate and audio tone, and applying that tone to the VCO. The FM-modulated output of the TCVCO could then be fed into a balanced mixer, perhaps one that uses the venerable MC1496. The mixer would multiply the FM signal with the 80m CW signal coming from the ARDF antenna. Coming out of the mixer would be the sum and difference frequencies for the FM signal and the 80m CW signal... plus the unavoidable harmonics. The harmonics might be enhanced by clipping the output of the mixer.

If the the VCO center frequency is chosen so that its sum or difference with 3.5 MHz falls on a subharmonic of a 2m band frequency, a 2m FM receiver should be able to detect the FM-modulated harmonic.

If the concept works well enough then it might eliminate the necessity of using a dual-band receiver for ARDF, since it would allow our 2m receiver design to work on both bands - only without the need for FM modulation.

Some potential parts:

1. MC1496 - https://www.digikey.com/products/en?keywords=MC1496BDR2GOSCT-ND

2. VCTCXO - https://www.digikey.com/products/en?keywords=1473-1543-1-ND

[Gerald Boyd]

Interesting concept. I think it’s worth investigating.

Need to think about it more. 

Jerry

Sent from my iPad

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[Gerald Boyd]

Thinking more since the new 2m receiver can do cw all we need to do is up conversation. FM may be less sensitive then CW.

Jerry

Sent from my iPad

On Mar 3, 2018, at 3:04 PM, NZ0I <charles....@gmail.com> wrote:

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[Charles Scharlau]

Agreed, regarding up conversion only, for our receiver. I also agree that the concept, whether for FM or CW, is probably significantly less sensitive. But generally 80m is noise limited anyway, with less need for high sensitivity. And I'm mostly thinking of creating a simple tool that would allow hams to participate in 80m without having to purchase or borrow an 80m ARDF receiver. It might not be good enough to make them competitive, but it would be a weekend project for someone wanting to give the sport a try, or use it for tracking noise sources, etc.

The MC1496 would likely require +/- 8V for mixer operation, based on the example circuits I've seen. So the antenna might require two 9V batteries so it can have positive and negative supplies. But that part costs less than $1. 

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[Gerald Boyd]

That’s a good point so maybe it’s like an 80 to 2 meter active attenuator that works with any FM ht?

Sent from my iPad

[Charles Scharlau]

That's the idea exactly. But rather than using a single diode to mix, the active mixer should be more sensitive and generate less intermod issues... I think.