Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 Ooo OK....sorry Daithi...I meant no offence.... So how does the choke effectively replace several inches of wire then.....?? Just enjoying a bit of light reading here..... Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 Basically all you're doing is coiling up a length of the wire so it's actually the same length (when the coil is unwound) and is tuned to resonance at 35 MHz On my ham gear I use a fixed length, but with a tuner unit that alters the reactance (with two capacitors and a coil (all variable) which means it works from 7 MHz up to 50 MHz - but it weighs more than my models ) Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 So given that the resonant frequency of my choke is 35MHz this is "A Good Thing" yes......i did notice some 10uH chokes that had a resonant frequency of 60MHz which confused me..... Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 That's what matters - that it's resonant at the required frequency - 60 MHz isn't an option. It's probably resonant with the stray capacitance between the windings Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 OK...so not all 10uH chokes are created equal then.......a point to watch. So far it seems to be looking good then......the proof of the pudding will be in the flying then..... Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted June 18, 2012 Share Posted June 18, 2012 From a bit a aerial theory from a long time ago, as I understood it, if the aerial is made shorter than one half wavelength, then it becomes capacitive, it should be purely resistive, so you add a bit of inductance to retune it. A series reactive tuned circuit is a low resistance. I’ve used a rubber duck, a short coiled aerial, on my tx for years with no trouble; I think this is more or less the same sort of thing, with the inductance being formed by the the coiled wire. Also as I remember, the DIY Micron radio transmitters came with a standard telescopic aerial, but with a centre loading coil. These short rx aerials were very popular a few years back, all the competition planes seem to be adorned with the little vertical whip aerials. As far as I remember, I don’t think there was ever any issues with these, and when you consider some of the distances these planes are flown at, and the secure environment needed, if there had have been a problem it would have soon resulted in some changes. So I’d say this will be fine at normal ranges. I inherited a miniature 4 channel Pro-Tech rx once, and the aerial was about one third the length of the normal rx. When the previous owner first obtained it we were very suspicious indeed, but it eventually proved to be an absolutely full-range rx, as in max eye-ball. I don’t know what internal gubbins, if any, there was, but this was 100% reliable. PB Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 Of course if you know any ham clubs near you that would have a MFJ antenna analyzer, it's just a matter of soldering a choke to a random length and trimming bits off until it's resonant Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted June 18, 2012 Author Share Posted June 18, 2012 Thanks Peter that makes sense.......I recall that the 35MHz JETI receivers had quite short aerials....shorter than other Rxs anyway..... In the absence of any analytical equipent I am going for the emprical approach..... But where I am confused is why two "identical" 10uH chokes can have such a different resonant frequency...one at 35MHz & one at 60MHz....seems odd to my simple brain.... Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted June 18, 2012 Share Posted June 18, 2012 It depends on the core Steve - if one is, say, ferrite, and the other is a different material, then the reactance will change. The wire diameter also makes a difference Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Author Share Posted July 2, 2012 Well a bit of testing has been done.....I took the base loaded aerial Rx plus an identical standard Rx to the local playing field & connected them to a servo & a battery & sent my 8 year old off walking with my transmitter (aerial down) with instructions to "keep wiggling that stick".......both Rxs responded well until he was out of shouting range......well over 50 yards. This would constitute a successful "Range Check" in my book..... So a "victim" has been selected for a flight test......my electric Mini Tyro... Isn't she pretty....? A closer look at the aerial......atop the read fuselage behind the wing.... So we're ready to go.....but I'm waiting for some nice flat calm weather in case she reverts to "free flight" mode.....obviously I will "range check" again with the motor running etc.....wish me luck..... Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 2, 2012 Share Posted July 2, 2012 Good luck Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Daithi, Perhaps (although it's rapidly becoming irrelevant in the model flying world) you could confirm something that I've long struggled to get answered clearly? Many people have told me that the length of an aerial is critical end to end in a straight line (i.e. held taut(ish) in a model) to achieve the designed resonance but I have long suspected that it is the electrical length which is critical. I appreciate that capacitance and inductance (amongst other effects) come into play if the aerial is coiled or twisted around itself but is it the case that bending the aerial a reasonable amount (say running one out to a wing tip and then to the tail) wouldn't materially affect resonance? Edited By Martin Harris on 02/07/2012 12:19:30 Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted July 2, 2012 Share Posted July 2, 2012 The length is the critical factor for resonance at frequency. The physical length depends on the velocity factor of the aerial itself (the speed of the electrons alters as it passes therough a conductor, so a correction has to be made from the basic formula of lambda = fc where lambda is the wavelength, f is frequency and 'c' is the speed of light). Resonance can occur at 1/8, 1/4 and 1/2 lambda. Adding a coil will convert a shorter aerial (say 1/8) into an electically longer one (perhaps 1/4) which will be more efficient Bending a whip alters the radiation pattern - if you consider the polarisation pattern, a 'bent' aerial will have one half parallel to the wave - but the other half will be vertical so while it may be resonant, only half of it is receiving the signal (it will still work and be resonant, just not as efficiently) By the way - I still call them 'aerials'. When I was an itty-bitty apprentice someone asked about 'antennas' and our tutor stopped, looked and said "Gentlemen - the plural of antenna is antennae and only insects have those. We use 'aerials'" Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Author Share Posted July 2, 2012 Just to dash off into a related area of physics, I'd always assumed that electricity travelled through conductors at the speed of light but I heard recently that this isn't actually true. Apparently electricity travels very quickly through a conductor but at someway short of C...is that right? Just to be a pedant I thought the formula was Speed = frequency x wavelength (lamda) so doesn't that make lamba = speed divided by frequency.....?? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Thanks Daithi, would it be reasonable then, in the case of a receiver aerial, to say that an aerial which had a significant bend at some point would be better overall as a compromise in a model aircraft, which is free to move in any axis as it would never end up presenting all of the aerial end on to the transmitted signal? (I'm struggling to visualise the effects of polarisation as I'm typing so this may be drivel!) BTW, I'm leading a one man campaign to get the word "wireless" back into general circulation (as in "I heard it on the wireless" - it's a far more woody word than that tinny term "radio", to paraphrase the great Monty Python... Edited By Martin Harris on 02/07/2012 14:54:30 Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted July 2, 2012 Share Posted July 2, 2012 Typo there Steve - it should have read c/f (lack of caffeine - me culpa) bending the aerial Martin changes the radiation pattern - sometimes for good, sometimes for ill (it can vary - I recall a CB aerial one time with a 45 degree bend at the top which actually reduced the standing wave ratio to almost unity - but made it more directional than it should have been). I sort of interchange 'wireless' and 'radio' depending - if it's telegraphy (Morse) it's WT - telephony (voice or data) is 'radio' - but I started off in the days of kilocycles and 'condensers' (and still draw a resistor as a zigzag - a box to me is an 'unspecified impedance' Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Author Share Posted July 2, 2012 I guessed it would be Daithi.... but we must protect the innocent of the parish mustn't we..... Like many things in life I guess aerials are a massive compromise between receiving the strongest signal from the most directions/orientations...... Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 5, 2012 Author Share Posted July 5, 2012 Well a bit more testing.....late evening yesterday the wind dropped right off and I thought a test flight might be in order. So I nipped up the farmers field just up the road. A quick range check looked good.....I got 30 yards away with the Tx aerial down & still full control of the model...even with the motor running. So a gentle lob into the gloom of a Lancashire evening & Mini Tyro was away.....I kept the model quite low & close by initially as I circled the field......no problems.....so I unleased the full 63 watts in the model & went for it (I know I always was a dare-devil.....). Gaining height & flying in ever greater circles took the model further & further away from me & soon it started to disappear into the low cloud. Still no sign of any glitches or lack of control. I circled as far as I dare just under the cloud base for over 10 minutes & the model responded perfectly.....I would guess its max height to be 200 feet or so but it could have been more.....or less.....I'm a bit rubbish at estimating the height of a model.... I flew the model right over my head several times as I thought this would be the worst possible orientation for the Rx aerial (pointed staight up & away from the Tx) & again with no problems. I would like to test the model a bit higher but couldn't last night because of the low cloud.....I might get to do this later tonight. But so far so good....... Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted July 5, 2012 Share Posted July 5, 2012 Daithi, the only reason we ever got inflicted with resistors drawn as boxes is because the draftsmen were too lazy to draw them properly. So they changed the standard...a year or two later and the point would have been moot because with CAD you don't have to redraw standard parts all the time. For readability you can't beat the old zigzag, it is distinctive, whereas a box could be anything, including an IC or a transformer winding since they wanted to do those as boxes too. John Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 23, 2012 Author Share Posted July 23, 2012 Well a bit more testing.......a nice clear day so the model was dispatched to as high as I could see it.....it was quite literally a speck, although being a small model (32" wing span) it probably wasn't actually that high...... Again no problems with control were encountered, the model behaved itself perfectly throughout the flight........I consider this to be a very successful practical test of the use of base loaded aerials...... Quote Link to comment Share on other sites More sharing options...
Boots Posted August 17, 2012 Share Posted August 17, 2012 steve from your first post I would think the atricle was an aerial amp (rf preamp ) which takes the weaker signal from a base loaded aerial and amplifys it, although you can boost the signal to its original strength it will be a noisey signal, maybe distorting the channel infomation, also the rx aerial input will have an agc (automatic gain control ) a preamp may not suit all agc's. suck it and see, we can ponder over electronics theory till the cows come home, if it works great, if not try a different choke, or wind a few coils yourself and try them Quote Link to comment Share on other sites More sharing options...
Dave Bran Posted August 17, 2012 Share Posted August 17, 2012 All this re-inventing the wheel isn't good for my health, my shoulders are hurting from all the shaking and my eyes will be red for days. I came to base loaded short Rx aerials on 35MHz in 2006 (which in itself was very late in the game) when I started flying indoors where the planes were so small that the normal aerial was a pain to try to "hide". Even then, the std aerial length on small Rx's varied enormously, from the not too bad Hitec HS05MS to the ridiculous half mile long (and extra thin fragile) Corona carp. OK, you might think Indoors equals no range requirement anyway, but try putting 15 and upwards models all on 35MHz in the air in a radio reflecting/distorting environment. Initially I bought from micronradiocontrols when they were about £2 each, later an ex-radio/tv repairman mate made them for me, now sadly deceased. I note they still list them at over double the price I paid, but are OOS currently. Indoors now? Well there is usually me and, errr, me on 35MHz, and most of mine have migrated to Spektrum anyway. (I even have five Spektrum Rx currently not in use) However, having inherited a large beutifully built collection of free flight, and not having the space to give them free rein, I'm starting to use my old 35MHz stuff on that, with transparent rudders and elevators added. Got to do something with my 6 all but redundant six memory 35MHz Tx's and a pile of Rx's!! So, this thread may yet prove useful to me, watching with interest if mild humour, keep going! Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted August 17, 2012 Share Posted August 17, 2012 Wasn't it quite common for RC turbine models to use whip aeriels so they could put the aeriel outside the model and away from all the metal and electronic gubbins. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted August 17, 2012 Share Posted August 17, 2012 all this reminded me of a set of circuits published in either RM or RCM&E, might have been the Digicon, I can't remember offhand. The receiver used a Motorola chip with an FET front end. The idea was that the FET buffered the aerial and detuned the input so any length of aerial would work. I did get good range with it using anything from 15-40" aerials depending on the model. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted August 17, 2012 Author Share Posted August 17, 2012 Thanks for the encouragement guys..... as I think has been discussed earlier in the thread these are not really aerial amplifiers but base loaded aerials.....effectively the choke fools the Rx input into "thinking" it has 40 inches of wire attached rather than a short length of piano wire..... Certainly the "testing" I have done with the one small model has been successful....it all just worked....I'm sure I will use this method again in my small light models but I'm not sure I'd trust a larger model to it...... Quote Link to comment Share on other sites More sharing options...
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