Receiver aerial, repair or replace?

[Robert H]

I have aquired a Futaba FP-R107N 35mHz receiver with a broken aerial lead. Is it safe to repair it with a good quality soldered join (followed by a range check) or does it really need to be replaced?

I have carried out the repair & done a range check, good response still received at 120 paces with the transmitter (Futaba T6X) aerial retracted & horizontal.

Any advice gratefully received by this newby!

Thanks

[AJ]

It needs to be the right length, other than that it's just a bit of wire so you should be ok to repair. Might be worth finding out why it broke, could there be any other damage to consider?

[Shaunie]

Just replace the antenna with a piece of similar wire of exactly the same length.

If you do not know the length I have two of these, I can measure them for you.

Shaunie.

[Chris Bott - Moderator]

Should be fine to join Robert, however, if it's been tugged on, the wire inside the insulation could be broken elsewhere. The most likely place is just where it exits the Rx.

[Robert H]

Thanks for all your replies. I have have the cover off the receiver & there is the rest of it looks fine.

Shaunie, it would be useful to know how long it should be, mine measures 96 cm from the case after my repair.

Any idea what sort of distance I should expect from the range check?

[Simon Chaddock]

Not sure about the exact length.

I have three 35mHz receivers that are not being used at the moment and with their original aerial wires.

Corona 4ch Synthesised 1035mm

Art Tech 6ch 990mm

SunUp 6ch 630mm

All measured from the case with the aerial wire pulled straight - gently!

Edited By Simon Chaddock on 25/10/2013 23:38:15

[Colin Naylar]

The ideal length of the aerial can be calculated by the formula wavelength = speed of light/frequency. The midpoint of the rc allocated 35Mhz band is about 35.2Mhz. The speed of light is 300,000,000m/s. Therefore wavelength is about 8.57m. A bit impractical, so it is more convenient to use an eighth wavelength aerial. ie 1.07m or 42inches. There may be more to it than that but I stand to be corrected.

[Bob Cotsford]

I think that figure has to be adjusted to take account of the aerial detector circuit - I remember that RCM&E or RM published a design with an FET front end that was not at all fussy about the length of wire.  I used it with anything from 10-40" dependant on the model being flown.

Where's Pete Christy when you need him?

 

Having said that, I don't think the 35MHz receivers are that fussy within an inch or so.  I think the Futaba ones I used to have were 39" ish - or a nice round metre.

 

Edited By Bob Cotsford on 26/10/2013 15:43:53

[Peter Beeney]

I’ve always had it that a practical aerial is 5% shorter than the calculation suggests it should be; this is because theory says that the electric field fringes out at the end, making the effective length a shade longer; so that would make Colin’s one eighth wavelength of 42 inches about 40. If you measure a standard 35 MHz receiver it tends to vary from around 37 to 40 inches; so I guess that it’s not that very critical; and as I remember, the Micron rx kits advised 39 inches; but I might be wrong about that…

I once repaired a shortened (broken) mini Fleet receiver aerial by hooking on a random long length, which somehow remained because I didn’t get round to adjusting it, this on a large powered glider which still had out of sight range; and then by the same token I was given a 4 channel Pro-Tech receiver with an aerial that seemed to be about 18 inches long which I treated with a great deal of suspicion in the first instance, but which subsequently proved to be a good as any other, faultless!

Another point is that in recent times I’ve read about FPV techniques, and in the instance where the model radio is a 35 MHz unit, in some cases the aerial length has been extended considerably to try and extend the range. Maybe this might be making a quarter wave length to increase the receiver performance.

It’s unlikely that a few soldered joints in the line are going to make very much difference, it’s working into a very high impedance, and unless a joint is so bad that it’s virtually open circuit it will go unnoticed. So as far as Robert’s OP is concerned, I’d personally have no qualms at all about repairing any aerial.

PB