Steve Hargreaves - Moderator Posted May 29, 2012 Share Posted May 29, 2012 Reading an old copy of RCM&E last night I glanced through Nigel Hawes article & noticed a bit on aerial amplifiers for 35MHz receivers. These allowed you to shorten the aerial wire to 4-6 inches or so & then plug it into the aerial amp & use a short length of piano wire as the aerial. Nigel pointed the reader towards GBDmodelaircraft.co.uk but the website no longer exists I believe. I guess they were an RF choke or similar....Anyone know if such a thing is still available? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted May 29, 2012 Share Posted May 29, 2012 I made one 3 or 4 years ago using a 10 uH RF choke (Maplin WH35Q), a small piece or two of heatshrink and 7” of very thin piano wire. I simply soldered the choke to the piano wire (I formed a small loop at the end for safety) and the other end of the choke to the first 4” of the remaining aerial wire and covered the choke with the heatshrink. The figures came from the internet and are supposed to be correct for 35 MHz – it's claimed that range testing before and after the mod. should show a reduction to no less than 85% of the normal range. Mine worked successfully in a smallish electric powered foam model and flight testing showed no glitching or range problems although I'd have hesitated to use it with anything heavier or faster. Edited By Martin Harris on 29/05/2012 12:05:15 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted May 29, 2012 Author Share Posted May 29, 2012 Thanks Martin....I've been doing a bit of surfing the internet & found some similar ideas to yours....see here Like you I would hesitate to use them in anything large but some small models seem to have taken over the workshop just now & I have a lot of redundant 35MHz Rxs knocking around..... Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted May 29, 2012 Share Posted May 29, 2012 Ah yes - that was my original source! Quote Link to comment Share on other sites More sharing options...
Wingman Posted May 29, 2012 Share Posted May 29, 2012 I used to use the ones on this page when I flew helicopters - never had any problems with them - however I don't seem to remember them being as expensive as that! Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted May 29, 2012 Author Share Posted May 29, 2012 Thanks Wingman......but not cheap as you say.........I can buy a 10uH choke for about 25p from CPC....that plus a bit of heatshrink.....lets say 30p all in.... Quote Link to comment Share on other sites More sharing options...
colin sayer Posted May 29, 2012 Share Posted May 29, 2012 On the subject of 35mhz receivers, where have all the new one"s gone, I have searched several sites for some and can"t find any for sale. there must be many people like myself with good 35 trannies who who dont see the need to change over yet. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted May 29, 2012 Share Posted May 29, 2012 That's the way it goes, I'm afraid, Colin - try finding a 27MHz receiver! As demand drops so does supply. There may be a "honeymoon" period when old stocks get shifted cheaply but I suspect that's well past now. However, Puffin are still listing 35 MHz Jeti gear so they may be worth contacting. Otherwise, if you're prepared to risk/test second hand ones there may still be bargains to be had from fellow members/ebay/swap meets etc. Edited By Martin Harris on 29/05/2012 13:28:47 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted May 29, 2012 Author Share Posted May 29, 2012 Lots of 35MHz Rxs still available Colin (try here) but as Martin says...c'est la vie.... Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted May 29, 2012 Share Posted May 29, 2012 I can vouch for the 35mHz Coronas from them. I have bought 9 of one type or another over the last couple of years. Very small and very light. I am not sure I could even see my models at the range claimed! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted May 29, 2012 Share Posted May 29, 2012 Posted by Steve Hargreaves - Moderator on 29/05/2012 13:25:39: Lots of 35MHz Rxs still available Colin (try here) but as Martin says...c'est la vie.... A scurrilous accusation - and you'll have the (other) Mods after you for using foreign language on the forum Quote Link to comment Share on other sites More sharing options...
colin sayer Posted May 29, 2012 Share Posted May 29, 2012 Thanks all, I bought a couple of corona syntherzised receivers recently but could"nt grt them to lock on to my trannie, but I wont give up with them just yet, it has to be me doing something wrong.Cheers. Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted May 29, 2012 Share Posted May 29, 2012 Just to inject a note of accuracy - loading coils on any aerial don't amplify (an 'aerial amplifier' is an active, powered device that literally amplifies the incoming signal - usually called 'boosters' or 'pre-amps' on domestic TVs in fringe areas). What they are selling are base loaded aerials (you can also top load or centre load). All it's doing is making an electrical length of about 1 metre (1/8 wavelength on 35 MHz) Centre loaded whips used to be very common on 27 MHz equipment (to shorten the aerial down to a reasonable physical length for an 11 metre transmission) Edited By Daithi O Buitigh on 29/05/2012 20:44:44 Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted May 29, 2012 Share Posted May 29, 2012 Xtals aren't being made for Futaba, JR, but can still get other sources that will work. As the RX's probably they've stopped production now, so apart from reserve stock and second-hand, that's it. Quote Link to comment Share on other sites More sharing options...
Alan Cantwell Posted May 29, 2012 Share Posted May 29, 2012 Flog you a few 138 recievers, owned by me from new, might even have a crystal to throw in, i dont flog rubbish,, by the way Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 Well a quick trip to CPC saw some 10uH chokes on the bench, a "victim" Rx & a bit of piano wire.... A few minutes with a soldering iron & some heatshrink produced this..... I've fitted a 2mm "bunch connector" at the end.....two reasons...it will allow me to swap from the base loaded piano wire aerial to the previously amputated wire as a comparison & I think will make it easier to install...we'll see.... Now I need to do a range check on the ground......I'll try it with the wire fitted & see how far I get as a "known" standard & then see how the base loaded version works.....if similar I will install the Rx in an (expendable) airframe & see how it goes..... More news as it comes in (but don't hold your breath.......) Quote Link to comment Share on other sites More sharing options...
Cyclicscooby Posted June 18, 2012 Share Posted June 18, 2012 Interesting stuff Guys.. Takes me back to my HNC Electronics days, but I tended to go selectively deaf when trannies and RF were mentioned... More of a Logic and programming girl me, hence PIC's... Thanks for saving me calculating values.. Interested to know the results of your test Steve.. Luv Chrisie.. xx Edited By Cyclicscooby on 18/06/2012 12:28:20 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 Hold on there Chrisie....you don't think I actually know what I'm doing here do you......? Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 I think that could have a disastrous effect on range Steve - using the formula of reactance = 2 x pi x frequency x inductance would make that a high reactance (somewhere around 2Kohms if my calculations are right) What just might work is a home made coil (cut, say, 18 inches off the trailing wire aerial and make a coil that matches that in length (wrap it and tape it round a dowel if it isn't rigid enough) and insert that in series with the remaining wire. The inductance should be a lot lower than the cored ones you're using. What is important is the electrical length of the wire (so that it's resonant at 35 MHz). You may have to do a bit of trimming to match it in so that the standing wave ratio doesn't go through the roof Edited By Daithi O Buitigh on 18/06/2012 14:46:44 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 You could well be right Daithi....I'm groping around in the darkness of my RF theory knowledge & just following the path trod by others....see here where a 10uH choke was suggested.... The spec for the choke indicates a resonant frequecy of 35MHz which seemed promising......or am I being hopelessly naive here........it wouldn't be the first time after all.....a little knowledge can be a very dangerous thing.... Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 Usually tuned circuiots are both capacitive and inductive (a cap in parallel with the choke) where it's 2 x = 1 / square root of inductance x capacitance You can find a calculator for it (and a few other things) here Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted June 18, 2012 Share Posted June 18, 2012 I'm not an RF expert but I built one of these a while ago (before 2.4 took off) and tried it on an outdoor foamy - got it to dot in the sky range (using an electric motor for power which often exacerbates any problems) and never heard or saw a glitch or any suggestion of range problems. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 H'mmm....thats seems to be part of the story Daithi but only part......my limited knowledge tells me theres more to it than that......after all look at what we are replacing here......about 500mm of thin wire..... My understanding of base loaded aerials is that they are there to "fool" the input side of the receiver that there is a different size of aerial than there really is.......would be good to hear from any radio HAMS or other such gurus.... Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted June 18, 2012 Share Posted June 18, 2012 Isn't (wasn't) that Daithi's day job? ...and a quick look at his profile: Been away from the hobby for a lot of years and now considering returning. Ham radio licensed (callsign GI7OMY) Edited By Martin Harris on 18/06/2012 15:19:24 Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 Posted by Martin Harris on 18/06/2012 15:15:34: Isn't (wasn't) that Daithi's day job? It was (and I'm also a licensed ham) That's exactly what they do - consider it as a coiled up piece of the trailing wire. The problem as I see it with the choke is it's probably ferrite cored - air cored would be better Quote Link to comment Share on other sites More sharing options...
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