Receiver antenna length, 27, 35, 40 50 and 72 mhz

Fellow R/C'rs: Correct me if I am wrong, but almost every receiver antenna that I have run across is usually one meter long...that is 39 nches, give or
take. I checked my 27 and 50 and 72 mhz receivers, both Futaba and JR and this bears out my thinking. The reason for this query is that a club member is now on the AMA jet team that goes to Hungary next month to fly. They will be flying on 35 mhz..which is in use in England and parts of Europe. JR is sending these flyers receivers and transmit modules on channels in the 35 mhz band. My friend has a jet that has a metallic finish. He is using a whip antenna and is concerned that the 35 mhz receiver antenna length will be different from the 72 mhz receiver he presently uses. I assured him that the one meter length is pretty much standard in the industry and see no reason why on 35 mhz it would be any different from the length of the 27, 50 or 72 mhz antenna on these receivers. Anyone know for sure about 35 mhz or care to comment? I promised him I would get back to him with what other modelers think..or know for sure. Thanks to all, Frank Schwartz AMA123 W4KFK in Hendersonville (Nashville area) TN
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On Fri, 01 Jul 2005 00:14:56 GMT, snipped-for-privacy@bellsouth.net (Frank Schwartz) wrote in

Well, you've bracketed the target frequency (35 mHz) in your samples (from 27 to 72 mHz), so you've probably already got your answer.

Ah. I see the problem.
This site <http://www.dxing.com/frequenc.htm says that this is the formula for wavelength:
wavelength = 300 / frequency in MHz
300 / 35 = 8.57 meters
300/ 72 = 4.17 meters
So the one-meter antennas come close to 1/8 wavelength for 35 mHz and close to 1/4 wavelength for 72 mHz. You might reason that his current whip should work OK as a substitute on the lower frequency if it works OK for the higher frequency. It should be in the ballpark.
Truly optimizing a receiver antenna is beyond my skills and equipment. You'd have to put some test equipment on the receiver itself to see what is really happening.
                Marty                 KC2NEB (lowest HAM rating)
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| This site <http://www.dxing.com/frequenc.htm | says that this is the formula for wavelength: | | wavelength = 300 / frequency in MHz | | 300 / 35 = 8.57 meters | | 300/ 72 = 4.17 meters | | So the one-meter antennas come close to 1/8 wavelength for | 35 mHz and close to 1/4 wavelength for 72 mHz.
For the record, the it's 1/4 wavelength that you're usually looking for in a whip antenna.
And your line of reasoning is correct, however if you look at the `serious' R/C radio equipment around the globe, on all the bands (except for the 900 or 2.4 gHz bands), the antennas are all about one meter in length. I can't claim to have checked much stuff outside the US, but in the US, 27 mHz, 50 mHz, 72 mHz and 75 mHz all usually has antennas right about one meter in length.
So if the antenna is one meter in length, and the frequency is not 72 mHz, there's two possible explanations --
1) there's a loading coil or other matching network that makes the antenna resonant at the chosen frequency, or
2) the designers have decided that the loss of range caused by not having a resonant antenna is acceptable.
I think it's usually #1 for most of the good R/C equipment. (I'm not talking about cheap Radio Shack-ish R/C `toys', or things like the Hitec Feather.) They probably stick with the standard one meter because 1) 3 meters for the 27 mHz band would be too long, and 2) it's easier to stock one length of antenna than several different lengths.
| Truly optimizing a receiver antenna is beyond my skills and | equipment. You'd have to put some test equipment on the receiver | itself to see what is really happening.
It's not so hard to make a good effort. In the absence of a matching network, the antenna should be about 95% of 1/4 wavelength. (The 95% is to cover the electrical length of the antenna vs. it's physical length.) The only real difficulty is determining if there is a matching network or not.
As for determining range, you don't need fancy equipment for that either, but it can make it easier and more accurate.
|                 KC2NEB (lowest HAM rating)
The level of your ham license doesn't really say anything about your skills. Many `techs' know a lot more than many `extras' ...
--
Doug McLaren, snipped-for-privacy@frenzy.com
If Barbie is so popular, why do you have to buy all of her friends?
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On Fri, 01 Jul 2005 03:38:23 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote in

It is a violation of netiquette to ruin theoretical arguments with the assertion of mere facts. :-P

AH! Now you're talking! I knew that there was some way of adjusting receivers to less-than-optimum antennas.

That was kind of my guess. As the Labatt bear used to say, "Crose enough!"

Makes sense. And it would be harder to mess up both in production and repair of the receivers if they all just have a standard-length antenna in stock.

OK. I can just about visualize what you mean.

That's definitely above my pay grade. :-O
I failed in my first attempts at RC in the 60s with Galloping Ghost equipment in part because I didn't understand "tuning" the receiver. The guys in the club I joined weren't able to help me, either, although they knew it was necessary. My used RX had no tuning core and I couldn't find the manufacturer to get one, so they wouldn't fly my airplanes for me. I went off and "flew" by myself. After a while, I got tired of wrecking and rebuilding the planes and went on to other hobbies, until I returned to RC in 1995.
I guess I'm still trying to fill in the blanks from what I didn't learn in the 60s.

I'm not one of them. I'm fully qualified for the lowest rating. :o)
                Marty
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How about someone who is involved in the project just asking the JR contact who is coordinating the equipment to check the length in their technical database? Then you don't have to rely on speculation.
Carrell

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Frank Schwartz wrote:

Its more thqan likley his whip is bottom loaded. It will need to be changed.
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No, I don't think his whip is bottomloaded....my main concern is to find out if I am correct in assuming that almost all r/c receivers, be it 27, 35, 50 or 72 all come with 1 meter long antennas, give or take an inch or so.... Frank
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Frank Schwartz wrote:

Not at all.
My JETI REX 4's come with less than 20", on 35Mhz, and the micron range of home built's state 'anyhing from 10" to 50" depending on range required' but advise sligthly retuning the input RF coil for a new length.
1m is a nice handy length that gives decent range without having to spend monety on a decent receiver, which simply wouldn't wear with th modelling fraterenity. That's all.
I fly with whatever bit of wire comes to hand frankly, but then I domn't expect full range and I am not flying a 200mph 12lb model that costs a couple of grand in front of spectators...
all this stupidity about 1m=tuned at 72Mhz is just that - stupidity. That appliesd to proper whips with ground planes in otherwise free sapce. You may get close to that in a transmitter: You certainly do not in a receiver. Wires are run up and down and around past various metallic objects.
IF your mate gets a decent reciever he should be up for 500-600m easily, which is a lot of range really. I reckon if you can get 60m on the ground with a collpased TX antenna you should be OK for a km in the air.
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The Natural Philosopher wrote:

Good stuff here. But I would be concerned about blind spots with a large metalic fuse.
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Hitec list a different length for 27Mhz, can't remember the exact length but it's listed in their receiver FAQ. Should be the same for most other brands.
--
The Raven
http://www.80scartoons.co.uk/batfinkquote.mp3
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I will go along with this comment. The whip is optimised to work with the USA receiver. The whip is *not* optimised to work at 35 Mhz. 39 inches is a practical length of aerial wire to use on a model plane. The 39 inch length can be made to work across all the different frequency ranges 35/70 Mhz, etc by means of a matching circuit in the receiver front end, however.. any after market aerial, such as a whip, hes to be capable of working with the compensatory circuit values in the Rx front end.
Reg G4N callsign
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wrote:

Perhaps I should also have stated.. The whip is somewhat different to a straight piece of wire. It will be more critical of loading.. it is itself a shortened, usually helically wound aerial.
Reg
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Futaba provide : 500mm as parts #0131 AM/FM27, #0133 AM/FM29, #0134 AM/FM40 for Futaba 2ch RX = short range use for cars/boats 1000mm as parts #0135 AM27 R102JE (long range) #0136 FM35, #0140 AM/FM29 (4ch+), #0141 AM/FM40 (4ch+), #0142 FM35 = R137/9), 1100mm #0146 FM40 = R137/9 and last but not least 1200mm for FM40 = R137/9. Hitec provide one replacement = #58011 and all except are 41 & 1/4 inches long = (1.0414m from pretinned end) JR = 1000 mm for most RX.
For further information refer to articles at http://homepages.ihug.co.nz/~atong under Radio Systems, Accessories, Alterations and FAQ
regards Alan T. Alan's Hobby Web Links http://homepages.ihug.co.nz/~atong

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According to Hitec, their 72Mhz receivers use a 42inch antenna. This is also the same length used on 36Mhz (for Australia). I would suspect that 35Mhz is close enough that you can use your standard length 72Mhz antenna.
--
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http://www.80scartoons.co.uk/batfinkquote.mp3
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