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2.4 Ghz receiver aerials

Started by planeman, August 17, 2013, 14:24:57 pm

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planeman

I have been having an interesting debate with a pal about 2.4 aerials which started when a receiver lost one of its aerials in a crash and he replaced it with a simple length of wire.
He maintains that this works just fine, pointing out that range seems unaffected and that there is nothing special about the commercial aerials fitted to receivers.
I remain very sceptical since on ground test with the other 'proper' aerial inadvertently disconnected there was little or no range when relying solely on the replacement wire.
We have agreed to differ on this but I remain concerned that any evidence that range is affected is likely to come after a crash.

Can anyone explain please just what is special about 2.4Ghz aerials that requires them to be made of the materials they are and indeed what exactly are they made of?

Phil_G

August 17, 2013, 15:13:48 pm #1 Last Edit: August 17, 2013, 15:38:10 pm by Phil_G
Your pal is half-right. Theres nothing special about a typical 2.4g rx aerial - just a short length of coax with the sheath removed for the last 30mm or so. I wouldnt solder a new bit onto the coax type of aerial, I'd strip a fresh inch-and-a-bit and just accept that the feeder is now shorter than it was.
If its a rx with no feeder, then its just a 30mm ish length of ordinary wire from the pcb.  However, if he's replaced it and theres no range, he's clearly done something drastically wrong - it should be as good as new. Maybe he's bridged the aerial input and ground when soldering the new aerial.  What rx is it?
Cheers
Phil
(PS The Hitec BODA is a bit different but its not magic  :af )

planeman

Thanks for the reply Phil.  The Rx is a Frsky and, to be clear, one aerial disappeared in a crash and has been replaced by an ordinary piece of 35mhz type cable.  The other aerial is an original 2.4 type.  Ground range checks are fine but I believe what we are now checking is a 'parkfly' type receiver with only one aerial doing the work.

I remain confused.  Why does 2.4 Ghz require coax type cable?  I assume it must require it because all 2.4 rx's to my knowledge fit it.  If that is true then surely a simple piece of 35mhz type cable is not right, otherwise manufactures would use it as it is cheaper than the 2.4Ghz arrangement.

Confused of Kimbolton!

Yoyo

It can be a simple length of wire - but only if it's directly attached to the pub and close to 31mm long. If you get something like an orangerx, that's what they use.

The co-ax that is still shielded acts like wire (actually a waveguide) so it's only the 31mm of unshielded core at the end which is the aerial.

A 35mm plain wire aerial IS still an aerial - but it's so badly matched to the frequency is being used at that it may as well not be there.

Aerial length is tied pretty closely to frequency, if you want it to be at all effective.
Doing what you like is Freedom
Liking what you do is Happiness

Phil_G

August 17, 2013, 18:35:40 pm #4 Last Edit: August 18, 2013, 21:31:01 pm by Phil_G
Most Frsky receivers have the aerials on U.FL connectors (like a miniature press-stud) so are easy to replace.
Presumably as your pal didnt do this we're talking about soldered-on aerials.
If he's made a good job of it, then from an RF point of view what he's done is ok, the coax is simply there to extend the aerial away into clear space, and many receivers omit the coax and just use a 30mm whisker directly from the pcb.  It sounds like you now have one of each type, which again, is quite common in commercial receivers.
Where the coax solders to the pcb there are two lands, one is the ground-plane to which the coax outer braid is soldered, then another very close by where the inner conductor is soldered. Its is critical that your new wire aerial goes to the 'inner' pad only.  The connections are very small and very close together.
I think possibly Yoyo may have misread the 35mhz thing (  :af ) , the wire you would use on a 35mhz aerial is perfectly ok to use as a 2.4g 'whisker' aerial, and actually 35mm of it isnt a bad match at all, typical manufacturers lengths being from 28mm to 35mm, depending on how much internal length there is on the pcb. Matching a receiver aerial is nowhere near as critical as matching one to a transmitter, although if you have telemetry its doing both.  Some of the Frsky V8R4's came with a very long aerial, about 55mm as I recall and therefore hardly resonant but in practise worked fine.
Its not really possible to say for certain if your rx is ok to use without seeing exactly what your pal has done, but if he has done the job right, it should be fine, as long as the new aerial is visible to RF.
Cheers
Phil

PS its bad practise to tape aerials down. They will rip out in a crash!
Tape two bits of drinking straw down, & loosely thread the aerials through so they can move if the rx moves.




Arceenut

August 18, 2013, 19:02:22 pm #5 Last Edit: August 18, 2013, 19:04:07 pm by Arceenut
Matching aerials to receivers and transmitters is quite a complex procedure. The length is dependent on the frequency and the number of 1/4 wavelengths it is designed for.  The length also depends on the matching used as the coax can also be part of the radiating element .  Matching for impedance is important, more so on a transmitter than on a receiver but is again a function of the receiver circuitry and the impedance of the co-ax can affect the efficient transfer of the signal from the aerial to the rx and from the tx to aerial.  To be sure you get the most effective system, I would suggest the replacement be an original manufacturer replacement or an equivalent type of co-ax with the right amount of inner lead exposed.
Lead, follow or get out of the way!

Phil_G

I understood that the replacement aerial was a whisker type straight off the PCB, but obviously if you're replacing coax it has to be an appropriate type, for receiver repairs I've used these  U.FL to U.FL patch leads, snipped in two (with the element bared of course):
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=685-1015 they're available in different lengths but the 300mm ones are handy. You have to be careful when stripping the braid not to nick the dielectric (the insulation of the 'inner' bit) . I roll mine gently under a sharp blade. Sometimes its easier to unwind the braid to snip the strands without damaging the inner.  This is all fine & dandy if you're confident of your knowledge & ability, otherwise as RCnut says maybe its best to get a commercial replacement. Cod do them.
I'd be very interested to see a close-up photo of your pal's repair job.
Cheers
Phil

Theaton56

It's not just a simple case of measuring a 1/4 wavelength but to get a best match, you would need to take into account the velocity factor of the antenna wire material.

Antennas & Propagation can be seen by some as black magic !

:)

Phil_G

In a transmitter I would agree, to keep the vswr down, protect the pa and ensure max transfer into the aerial. But its simply not as critical in a receiver, the front end being of necessity so broadly tuned.  For a simple whisker aerial such as on a park-fly rx, 30mm ish of ordinary multicore wire is fine. On coax antennas, if you check coax specs between various manufacturers the vf varies very little.  I very much doubt you'd get a vf spec for multicore hook-up wire!   :af
But back to this repair - I'd still like to see a close-up of this fix!
Cheers
Phil

planeman

Sorry, no pic available but the 'fix' is just a length of flex the same overall length as the other, proper aerial.  The rx does seem to work as evidenced by recent flight tests but I remain sceptical.  I suspect the rx is operating with just the one proper aerial, rather like a park flyer.  my pal swears he has fixed it!

Phil_G

Quote from: planeman on August 22, 2013, 22:40:53 pm
I suspect the rx is operating with just the one proper aerial, rather like a park flyer.  my pal swears he has fixed it!
I cant see any reason for you to think that. You'd have to look with a scope at the RF switch select line to tell.  If its a good repair, why would it stop switching? are you thinking that the original aerial is doing all the work? Thats easily tested - just shield it, and do a range test. In open air the original coax aerial and the replacement whisker should give similar results.  That is, as has been said before, if he's done a good job!
Cheers
Phil

tomkfly

Is this length of wire 30ish mm long, or a length of wire the same length as the coax plus the bared end? If, as I susspect, you are talking the second option, then it is definitely wrong.

Tom
When you change the way you look at things, the things you look at often change.        www.deesidemac.co.uk
Gravity, thou art a heartless bitch

Phil_G

Quote from: planeman on August 22, 2013, 22:40:53 pm
... just a length of flex the same overall length as the other, proper aerial.
Thats the first time you said 'overall' length!  Tom is right, but all the previous replies have talked about a 30mm whisker, are you saying your pal used a much longer length than that?

planeman

Sorry for not replying, been offline due to other urgencies. Yes the repair aerial is the same length as the genuine aerial i.e. coax plus the bared end.  It just got a bit incidental today since the aircraft it was fitted in crashed with all the evidence of loss of signal!

Phil_G

Quote from: planeman on September 05, 2013, 17:06:48 pm
Yes the repair aerial is the same length as the genuine aerial i.e. coax plus the bared end. 

Then I'm afraid your pal is not as clued up on RF as we thought he was.   The repair he did was flawed, and his advice was simply wrong.
Sorry it cost you a model to find out!
Cheers
Phil

tomkfly

September 05, 2013, 19:28:37 pm #15 Last Edit: September 05, 2013, 20:27:10 pm by tomkfly
The aerial is only the bared end. The coaxial part is shielded and only serves to carry the signal from the aerial to the reciever, in the same way the coax into your TV carries the signal from your roof array to the TV.

Tom
When you change the way you look at things, the things you look at often change.        www.deesidemac.co.uk
Gravity, thou art a heartless bitch

Saxon

Quote from: planeman on September 05, 2013, 17:06:48 pm
Sorry for not replying, been offline due to other urgencies. Yes the repair aerial is the same length as the genuine aerial i.e. coax plus the bared end.  It just got a bit incidental today since the aircraft it was fitted in crashed with all the evidence of loss of signal!


The good news I suppose is that he saved 99p  :banghead:

http://www.giantshark.co.uk/product/169996/frsky-aerial-for-rx-15cm

John