Peter Christy

I usually find that if the CofG is too far aft, I regret leaving my bicycle clips at home.....

Has anyone else had the problem of print shops refusing to print - or even copy - plans, however old, if they contain a copyright message? Mine wouldn't even consider copying my existing (tatty!) plan from a company long out of existence for this reason! -- Pete

Nigel is right. The peak voltage is quite a bit lower on NiMhs compared to NiCads, and a NiCad charger is unlikely to detect when the batteries are fully charged. Is it not possible to rig your model charger to charge the new pack? You might just need to make up a suitable charging lead. -- Pete

Still trying to figure out how it would be possible to fit one of these into my single-channel Sharkface or Sub-Mini alongside the rubber driven escapements! 🤬 Like most legislation, it is only any good if its enforceable. This strikes me as being something that is - for all practical purposes - unenforceable, at least, until after the event! Since I generally only fly at recognised sites these days, its not high on my priority list. Having said that, the cost of the FrSky unit, alluded to above, sounds realistic. No doubt some will be cheaper and some more expensive, but i can't see it being much different. More to the point, even that wouldn't fit in some of my models. Obviously something dreamed up by someone interested in appeasing political interests rather than for any practical purpose. Not something I'm prepared to waste time on! -- Pete

You have no idea how true that is!!!

I have a classic car: Had it 16 years now. (Wasn't like that when I got it!) Been all over Europe in it, one trip to Interlaken (Switzerland) and another to Ballenstedt in the old East Germany for the model helicopter championships. Never missed a beat on either trip. I do all my own routine maintenance. Also play baritone Ukelele in a band, doubling up on bass guitar when required! 😄

Not sure I entirely agree with this. A lot of Txs in days of yore used folded aluminium cases, which make a pretty good Faraday cage. They also improve capacitive coupling between the RF sections and the pilot, improving (slightly) the effective radiation from notoriously inefficient telescopic aerials. When plastic cases became the norm - usually from mass produced sets - it became much, much easier for outside signals to break in. Eventually, as technical regulations tightened up on spurious emissions, they started treating the inside of the plastic cases with a conductive coating, to try and replicate the Faraday cage effect. They also had to screen the electronics better at circuit board level, for much the same reason. So yes, there was a brief period when older Txs were more vulnerable, but the really old ones are pretty much bullet proof! -- Pete

Strangely, the incident I witnessed (referred to in my earlier post) also involved a Multiplex Tx, though on 35 MHz and using a JR receiver (for some strange reason)! I've regularly heard the speakers on my computer at home make a terrible rasping rattle when my mobile rings, and it was also something I experienced at work if a phone rang nearby. As I said, its the PEAK power that matters, not the average. It is unlikely to break through into the RF sections of a transmitter, which should be adequately filtered, but the encoders are another matter. Would probably be exacerbated by the use of a buddy box lead, which would act as a pretty good aerial feeding into the encoder! Airplane mode on the phone should be OK, but rather defeats the object of having a phone in the first place - unless you're just using it as a camera. Like I said originally, and others have confirmed in this thread, I believe its a problem pretty much consigned to history - but you can never be absolutely sure. And remember, ALL our systems are built down to a price, not up to a standard! If our systems were built to commercial aircraft standards, we'd be paying 10x the price for our gear - may be more... -- Pete

Actually, that's only partly true. Its not the frequency they operate on that matters, its the peak envelope power of the pulsed signal that mobile phones emit. Haven't you ever heard a mobile phone breaking through into audio speakers? It sounds like a harsh digital rattle. The average power output is low, but the peak power is surprisingly high. Mobile phones constantly vary their power output depending on feedback from the cell tower. Since flying sites tend to be in rural locations - a long way from the cell towers - the power is likely to be higher than in an urban environment. It is possible for a mobile phone to break through into the encoder circuits of a Tx, but most manufacturers have now wised up to this and improved the screening in the Txs. A decade or so ago it was a real problem, and quite a few models were destroyed by it. I believe the BMFA issued a recommendation not to have a phone on you while flying, but to leave it in the car. Having said all that, the only time I witnessed this happening - and it could be repeated, so there was no doubt - was back in the 35 MHz days. I haven't heard of it happening to 2.4 GHz gear as they generally have to be better screened anyway to get approved. But it is theoretically possible. However, I would have expected the result to be random twitching of ALL controls, or a total freezing of the system - which should be followed by a failsafe. The symptoms described by Rocker do not sound like phone interference, but it might be worth leaving the phone in the pit area or car while flying... -- Pete

Phil's explanation is pretty close. Just as at 27/35MHz a single conversion receiver (455KHz IF) will have poor image rejection (910KHz above or below wanted frequency, depending on whether the local oscillator is above or below the wanted frequency), so 10.7MHz is pushing it a bit at 459MHz. However, I've never experienced an image problem on 459MHz using a 10.7MHz IF, just as few suffered from the same problem using 455KHz on 35MHz. I also used a crystal filter for the first IF, to protect the the 2nd IF mixer. We used a Toko clone of the 3357 receiver chip for the 2nd IF, as this was less susceptible to adjacent channel overload than the basic 3357, but neither could compete with the Siemens SO42 mixer we used at 27/35MHz. However, the Siemens needed another matching chip (SO41) for the IF amplifier and demodulator, and simplicity/size considerations won out! Regarding the triple superhet, it enables the use of a thing called a Wadley Loop for drift cancellation. ( https://en.wikipedia.org/wiki/Wadley_loop ) It shouldn't really be necessary if both local oscillators are crystal controlled, however bearing in mind that the 1st IF oscillator is operating at 1/6th (in my case) of the final wanted frequency, some small drift is inevitable. I have no idea if this is how the Cotswold worked, as I've never got my hands on one, but it would be a reasonable way to go. Indeed, I only think it used a triple superhet as it was mentioned in a report on the system at the time. It may be duff gen! Drift could also be fixed by using an Automatic Frequency Control, but in my case, I used exactly the same circuit for both the Tx and Rx oscillators. I reasoned that this would probably ensure that they both drifted together, and allowed for a simpler circuit. It seems to work well! I also could have used 21.4MHz as the first IF in my receivers, as the necessary filters were available, but they were more expensive. Any 459MHz set is going to be more expensive than a comparable 35MHz, so cost was a fairly important consideration for a commercial product, but not at the expense of reliability!

Speaking of car racing, Phil Greeno's used to be my local model shop at one time. I remember him buying a Reftec when they first came out, to get away from the 27MHz clutter. I visited his shop one day to find him with the receiver in pieces following a loss of signal, and showing me all three legs of one transistor sliding through the solder joints quite freely! He took it to Monaco for an international race meeting only to discover that the local TV channel was on 459 MHz! A few years later, I bought one of the later Reftecs from bankrupt stock, after they went bust. I wasn't impressed. I tidied up all the solder joints - or thought I had - but it still crashed my son's trainer one day. Luckily a bush broke the fall, and the model was intact. On pulling the receiver out, I could hear something rattling inside. Yes, another component had fallen off the board due to poor soldering.....! I re-soldered the receiver (again!) and passed the whole thing on to someone who wanted it for a boat. At least it couldn't do much damage in that!

No! The Reftec was the first to market, and paid the price! It was an "interesting" design. The receiver was a double superhet, but only used one crystal for both IFs. This meant that the IFs were slightly different depending on which end of the band you were operating in, so the receiver was fairly "sloppy" to accommodate this! Using a 1st IF of 27MHz and putting the receiver in an unscreened plastic box probably wasn't a wise idea either, and the receiver usually went wild when a model was being carried out through the pits - surrounded by 27 MHz Txs! The build quality wasn't great either, with cold solder joints that allowed components to shake off the board not uncommon! The Cotswold was a completely different kettle of fish!. I believe the receiver was a triple superhet, though I've never got my hands on one. I do have a Cotswold transmitter, and it is VERY well made - built up to a standard, not down to a price. My own 459 MHz (sold by Mick Wilshere, World Engines) sets sat somewhere in between, using as many "off the shelf" components as possible to try and keep the price down. I think our receivers were the smallest too by some margin. The first generation fitted inside Mick's standard 27 MHz receiver case! We also did a "Mini" Rx, about half that size, thanks to the efforts of Peter Valentine, of "Lark" helicopter fame. My original prototype still files occasionally, in a Lark!