Mode Zero

As some will know, in my conversions of retro sets I like to make my own 2.4GHz antenna and hide it within a fake 27MHz frequency pennant so the tranny looks as original as possible......,.

I check them on an RF Vector Impedance Analyser before fitting them and the latest is pretty good with a VSWR of less than 1.1 at 2.4GHz but the curve has pretty steep sides so it doesn't have to be far off 2.4 before the VSWR goes shooting up.....

Are our current sets are on EXACTLY 2.4GHz or does the 2.4 refer to a frequency band and our sets fall somewhere within that band but not necessarily on exactly 2.4GHz?

Paul

Hi Paul, no, its a (nominally) channelised band carrying broadband signals so really there is no one fixed frequency in the 35Mhz crystal sense.
The DSSS systems like DSM2, DSMX, FASST etc are wider bandwidth - true broadband, than pure hoppers like Frsky, Futaba FHSS, Flysky etc and they all (except the original DSM and some 'toys') use multiple channels which are chosen in different ways, some choose randomly, some by listening & picking quiet channels, some generated from the unique GUID. The chosen channels can be anywhere within the band, some re-use the same channels all the time, some re-use the same channels unless they're busy, some choose new ones every switch-on. If you could 'hear' it our transmitters would sound like random noise. Ideally a 2.4g aerial should be flat 1:1 across the entire allocation of 2400 to 2483.5 MHz but you're unlikely to achieve that and as ever its all a big compromise Probably the best you can do is to tune it midway, around 2440 Mhz. Resonance is naturally 'peaky', in aerial design such as for example television, as much effort goes into achieving a flat response over the entire TV band as goes into producing a high gain

Many thanks Phil, I can easily tune it to 2440 but if I can't make it flat, is there any way I could make it less "peaky"?

Paul

Just how "peaky" is it? As long as it stays under 1.5 across the band, you're good to go. Indeed, you probably wouldn't notice the difference if it went up to around 1.7 or so at the edges. Personally, I would worry and start taking remedial action if it reached 2, but as long as it remains below around 1.5 across 2400-2483.5, I wouldn't worry.

You make them less "peaky" by reducing the "Q" (or "Goodness Factor"), but this will also decrease the gain of an antenna. Most RF boards are designed to work with an antenna gain of 2dB, and produce 60mW output. 60mW + 2dB = 100mW ERP (Effective Radiated Power), which is what we are allowed - and is more than adequate for our purposes. Most 27/35 MHz sets failed to achieve 100mW despite having more than 1 watt output stages. This is all down to aerial efficiency. The transmitters I designed had 1 watt output stages, but only measured around 80 mW ERP.

ERP is notoriously difficult to measure, and I only know these figures from the reports of the type approval people!

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Pete

Thanks Pete,

"Just how "peaky" is it?"....... I would say "very" but by tweaking/re-tuning it to somewhere near the middle of the band, as Phil suggested, it now seems to be below about 1.6 over the whole band so I guess that is about as good as I'm going to get......
Many thanks both for your input.

Paul

Yeah, that sounds pretty fair to me. Certainly not enough to worry about!

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Pete