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coiling 72MHz antennas?

Old 05-22-2015, 02:38 AM
  #1  
FlyWheel
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Question coiling 72MHz antennas?

I've heard this can be done, so long as the coil isn't too tight and it's not wound around anything conductive, like metal or CF. If this is true, I would like to get all of the antenna inside the fuselage of my newest plane, but the antenna is about 6" too long (so close!). So I was wondering if it could be coiled without significant signal loss, and if so, which method would give better results?

"Base loaded"; The section of wire closest to the RX is coiled, the rest left straight (like the magnetic CB car roof antennas).


fully coiled; a loose, open coil along the entire length of the wire, no tighter than absolutely necessary to fit.



P.S. Don't even think of suggesting 2.4GHz I'm not in the least bit interested.
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Old 05-22-2015, 02:42 AM
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fhhuber
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Anything other than straight will reduce useful range.
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Old 05-22-2015, 03:41 AM
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What I did back in the 27/72 days - clipped the antenna X number of inches, soldered end to 1/16" brass tube about 1" long, mounted tube in top of plane and jammed a 1/32" music wire X inches long (same as antenna clip off). Now I had a vertical whip antenna and had mondo range.
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Old 05-22-2015, 04:47 AM
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My choice would be to do a perimeter of the wing.

Take it out along the LE and just follow the wingtip. Come back along the aileron hinge line. Tape in place.

Straight is best... as straight as reasonably possible if you don't want it hanging out the back of the plane.
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Old 05-22-2015, 04:57 AM
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FlyWheel
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A good idea, alas, the wing is not a permanent part of the plane. However, it gives me a possible idea, extend it along the fuse, then up the LE of the tail (or one of the H-stabs).

Thanks, I'll have to do some measuring tomorrow!
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Old 05-22-2015, 05:20 AM
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Who says an RX has to be in the fuselage?
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Old 05-22-2015, 05:46 AM
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I used to install a plastic tube in the plane, close to the surface, and stuff the antenna down that. Just don't do it under a metallic covering, even metallic monokote. Could be in the body or wing.
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Old 05-22-2015, 07:29 AM
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kyleservicetech
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Default Receiver in the Wing

Originally Posted by FlyWheel View Post
I've heard this can be done, so long as the coil isn't too tight and it's not wound around anything conductive, like metal or CF. If this is true, I would like to get all of the antenna inside the fuselage of my newest plane, but the antenna is about 6" too long (so close!). So I was wondering if it could be coiled without significant signal loss, and if so, which method would give better results?
.
That 72 Mhz antenna is actually tuned to 72 Mhz when fully extended. Coiling the antenna might work, but you absolutely have to check its "Tuning" after doing so. Checking tuning requires specialized equipment. You're very likely to reduce the range. Significantly.

Think about the range of your 72 Mhz transmitter with the antenna fully extended. That range is in the area of several miles. Collapse that antenna, and your range drops to a hundred feet or less. The same would apply to the receiver antenna. I've seen two crashes now where the modeler installed his 72 Mhz receiver, and forgot to range check it, and forgot to un-coil the antenna as received in the box and stretch it out. Both models represented a lot of very detailed scale craftsmanship. One was an 7 foot wingspan 7 foot model with four electric motors. The modeler had spent several years building the model. Completely wiped out in the crash.

As for the receiver location, I've installed the receiver, along with a pair of 5 cell A123 battery packs, two Hyperion 450 Watt motors, and matching ESC's in the wing of my TwinStar. Only the elevator and rudder servos are located in the fuselage.

(Of course, that receiver is the "Dreaded" 2.4 Ghz Spektrum unit, but same could work with the longer 72 Mhz receiver antennas.) I've got about 100 flights on it now, and the combination has been absolutely flawless.
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Old 05-22-2015, 08:36 AM
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As the others have said, anything but straight reduces range. The fully extended length is exactly quarter the wavelength of the 72MHz radio signal, so it's designed to be straight to work. A coil is likely to reduce range dramatically.

I know you said you weren't interested in 2.4GHz, but especially for small models, why? It's the only way to solve this problem.
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Old 05-22-2015, 09:47 AM
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There's no reason why an anteena cannot route along fuselage and then up tail or across stab ....

But to be quite honest - for years I had my 35 and 27 antennae hanging out the back with no trouble at all. The only time it would be bad is with a pusher model, then you lead it forward and let it fold back with at least fuselage width between the lengths.

Nigel
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Old 05-22-2015, 10:01 AM
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I stumbled accross this page: https://www.homefly.com/reference/antenna.htm

It covers design of loading coils, also this statement is of interest:
The longer the antenna (up to 987mm for 72MHz), the better it will work. Put as many bends in the antenna as you like as long as no bend exceeds 90 degrees and the antenna does not double back on itself.
So based on that statement the loose spiral wound around a plastic tube, as shown in the opening post should work fine. As long as it doesn't double back.
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Old 05-22-2015, 10:05 AM
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Potential for up to 3 miles on 72 mhs with no mods... but generally can't expect more than 1.5 miles reliably.

Re-tune the RX for full wave antenna (and put on the longer antenna) and you can get several miles. (Yes you have to re-tune it)

Fold the RX antenna in half any you might be cutting the useful range by 75%. You don't have to cut it to severely affect its range.

Anything other than straight will have some effect... How much would require extensive testing for your individual case.
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Old 05-22-2015, 03:55 PM
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I had used a 6" long plastic tube to wrap (coil) the 72mhz antenna for this heli. It increased the signal reliability strength by at least 300'.......never flew further than what my LOS was for the size, about 80' vertical and 600' horizontal.....the coil application helped a lot.

Last edited by pizzano; 05-22-2015 at 04:21 PM.
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Old 05-22-2015, 05:26 PM
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P : Interested to know how you can say coiling your antenna helped ?

Did you SWR the antenna .. if so HOW !

Did you test signal strength before coiling / after coiling ... at extreme distances ?

Given that you only flew LOS with what is effectively a small heli - even folding the antenna would probably have worked without noticing any problems.

Nigel
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Old 05-22-2015, 07:36 PM
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Relocating in the process of coiling (instead of what he was doing before to keep it out of the rotors....) could easily give an increase in useful range.

Re-aranging the coils evenly spaced over the whole tube might help too. There's effectively a fold-back there.
But it ain't broke... don't fix it. got the range you need , then you're fine.
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Old 05-22-2015, 08:30 PM
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Originally Posted by solentlife View Post
P : Interested to know how you can say coiling your antenna helped ?

Did you SWR the antenna .. if so HOW !

Did you test signal strength before coiling / after coiling ... at extreme distances ?

Given that you only flew LOS with what is effectively a small heli - even folding the antenna would probably have worked without noticing any problems.

Nigel
Nigel.........

The heli was a 300 class fixed pitch.........I tested the signal strength (reliability) and distance (ground based distance) three different ways.

1) The initial flights were accomplished with the antenna wire wrapped around one of the landing rails....like the manual suggested.
2) The second test was allowing the wire to just dangle below the heli....I had added a little weight to the last 2" of the antenna with wrapped electrical tape.
3) The third and final set-up, like shown in the photo, is the one that worked the best.

The ground based tests were performed at a soccer complex where the field distances are laid out....300' being the length of one pitch, 600' being back to back fields (goal to goal).......LOS vertical distance of approx. 80' due to field light post heights of 40', which we doubled in flight.

The first two tests applied gave us intermittent signals after around 450'......the last test provided no signal interruption through and past the second soccer field......all performed at the same site, back to back.....so environmental conditions would not come into play.......there were no other RC devices at the site during our testing.

I've never flown that particular heli more 650' away horizontally or 100' vertically, due to it's size and lite weight..........the reason I tested the signal issue, in the first place, was due to constant glitches I was getting with the suggested (manual) antenna install........I had seen a similar antenna set-up on a gasser at the club..........but was a newbie at the time and didn't ask a lot of questions from guys who were a little indifferent to electric fixed pitch fliers........lol
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Old 05-22-2015, 09:26 PM
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My HiTech Mini 6S 72mhz receivers were specifically provided with a "bobbin" to coil the antenna and shorten it. Never gave me a lick of trouble, even when specking out a 2m sailplane using it.

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Old 05-23-2015, 11:04 AM
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I want to do some indoor flying and whlst I have my 2.4 gear I also have a tiny Fagaio? 36 mHz full range Rx and Tx.
Now, being 1 meter long, the antenna is going to look silly dragging behind the plane and also the possibility a prop tangle could ensue.
So... based on my limited knowledge from the old 27 mHz days and 1/4 wave antennas, is there a chance I can shorten it to 1/4 wave. I am only looking for 50 meters of range here and being a Rx it is not likely to burn out because it has a short antenna.
Anyone ??
Thanks.
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Old 05-23-2015, 03:03 PM
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Originally Posted by Panther View Post
I want to do some indoor flying and whlst I have my 2.4 gear I also have a tiny Fagaio? 36 mHz full range Rx and Tx.
Now, being 1 meter long, the antenna is going to look silly dragging behind the plane and also the possibility a prop tangle could ensue.
So... based on my limited knowledge from the old 27 mHz days and 1/4 wave antennas, is there a chance I can shorten it to 1/4 wave. I am only looking for 50 meters of range here and being a Rx it is not likely to burn out because it has a short antenna.
Anyone ??
Thanks.
Think on this .... Range Check mode is typically 1/10th or less power Tx RF .... and that gives over 100m usually ...

So I would basically fold the existing antenna and use it. No need to cut it. Indoors is not outside and risks of flyaways ...

Nigel
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Old 05-23-2015, 06:29 PM
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Some of the micro 72 mhz rxs used half length antennae.
Range was drasticly lower than normal length. In some cases you got more range from 27 mhz.
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Old 05-23-2015, 06:35 PM
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Originally Posted by fhhuber View Post
Some of the micro 72 mhz rxs used half length antennae.
Range was drasticly lower than normal length. In some cases you got more range from 27 mhz.
27 was standard in UK for many years until we got 35 allocated.

27 was OUT OF SIGHT range as was 35 ... our problem was the CB guys when they fitted boosters to their sets. CB band was in the upper 27 part of our model band ... so as long as you stuck with the lower 27 freq's - you were ok ... EXCEPT when some idiot decided to switch in his booster on his CB set .... they were so 'dirty' they literally wiped out entire widths of 27 !

Nigel
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Old 05-23-2015, 06:36 PM
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Default Antenna length

R/C Antenna Experiments
Does Rx Antenna Length Really Matter?


FWIW, I have a Weasel slope plane, full length 72" antenna is coiled around a plastic 'bobbin' as describe in an earlier post, with about 8"-10" running straight back and trailing just a bit. I've had it out as far as I care to (spec'ed out) and it seems to work OK. Berg 4-ch RX. I think the Berg's work better than many. I also have a couple GWS 4-ch that come stock with a 39" antenna and on a pretty wide open (Slow Stick) fuse I've had trouble with signal at 300'. JR7202 TX-yep, still on that same old system and it works fine.

Buddy uses a similar Berg setup on his Weasel. He kept losing a signal at around 150'. We found the antenna had broke at the RX solder joint, that is no antenna at all. Soldered back on, works fine.
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Old 05-23-2015, 07:06 PM
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27 for USA is the undesired stepchild. Limited to extreme low power TX and thus stupidly low range. Some 27 mhz stuff can't even work across a typical living room.
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Old 05-23-2015, 08:18 PM
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Originally Posted by fhhuber View Post
27 for USA is the undesired stepchild. Limited to extreme low power TX and thus stupidly low range. Some 27 mhz stuff can't even work across a typical living room.
Given the proliferation of CB in USA .... surely 27 is a daft freq. to use ?

In UK CB radio was illegal but many CB sets were brought in and Tx'g on 27. We had hell of a time while Govt sorted it out ..... of course typical Govt didn't listen to anyone and tried to legalise CB on a daft 459 freq. No-one was interested as Europe and USA were CB 27. Finally Govt realised they were on losing battle and legalised 27 on CB with 35 allocated to model flying. This left the 'surface' models still on 27 ..... took ages to get that sorted but finally 40 was allocated to surface !

BUT all freq's for models ... 27, 35 and 40 were full range allocations.

Nigel
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Old 05-23-2015, 10:56 PM
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People that show up with certain 27 mhz system RTF packages... I will replace their radio with a 2.4 ghz from a RTF package if I have one with me.

I buy the RTF versions mainly due to the RX ready not being in stock and impatience. I added it up a few times and the effective cost of the TX + RX comes out to about $5 to $10. I spend that in gasoline for one or two trips to the flying site.

Currently I have one of the give-away 2.4 systems left.
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