Repost: Hidden 2.4ghz transmitter aerials

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Phil_G
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Repost: Hidden 2.4ghz transmitter aerials

Post by Phil_G »

Reposted from 2016, which was in turn a repost from 2012 :D

Internal aerials are sometimes very convenient and dont adversely affect range if done properly.
As an example heres my 6EX home-made aerial inside the plastic handle. This set is used exclusively for soaring, often at extreme range - no problems, and actually fewer RSSI pips than I had with the standard aerial, possibly because of the 'ideal' orientation. Originally I used a 'gutted' plastic aerial but once I'd realised how simple they are I now make my own from coax & brass tubing. Not having a fragile external aerial is a real boon when battling through 6' ferns to retrieve a downed soarer!

Untidy inside:
IMG_20170417_175656366.jpg
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But tidy outside!
IMG_20170417_175854416.jpg
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aerial.jpg
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Cut the tubing to the exact length first, by rolling under a new scalpel. clean and very lighty tin the rim at the 'whisker' end.
Cut the cable with a couple of inches of excess length, then thread the cut cable through the tubing BEFORE stripping and fanning the braid.
Strip & fan out the braid like flower petals the solder the braid around the tinned rim quickly and carefully to avoid damaging the dielectric.
Trim off any whiskers, and finally cut the 'inner' to length. Measure twice cut once!
Try to accurately follow the measurements - they have been carefully optimised for minimum vswr at 2.4ghz.

4mm OD brass tubing:
http://www.ebay.co.uk/itm/Brass-Tube-2m ... 0813916387


Here's a genuine Frsky transmit module aerial, the whisker is to the left. The short stub to the right
(30 to 31cm) is the remains of the feeder where I cut the connector off.
IMG_20181112_195901.jpg
Cheers
Phil
HenkB
Posts: 44
Joined: 01 Feb 2019, 11:19
Location: Harderwijk, The Netherlands

Re: Repost: Hidden 2.4ghz transmitter aerials

Post by HenkB »

Hi Phil,

What is the theory behind the 24 mm long, 4mm OD brass sleeve at the end of the coax lead? It is connected to the inner metal shielding of the coax lead, I understand, which is longer than the brass sleeve. Can't see any advantage coming from that but then; I'm not very much an electronic geek ;)
Martin
Posts: 744
Joined: 16 Feb 2018, 14:11
Location: Warwickshire

Re: Repost: Hidden 2.4ghz transmitter aerials

Post by Martin »

Radio ham half-wave dipoles have balanced feed wires, not coaxial - and if the feed from the transmitter is unbalanced (coaxial cable) then we use a balun near to the antenna to convert from unbalanced to balanced.

But of course, with our model transmitters, we're running tiny amounts of power down very short lengths of cable, so the unbalanced feed doesn't really matter. These 'tube around the coax' antennas are what's inside all the short "plastic" antennas that commercial transmitters are supplied with - and they're also used on WiFi routers and similar. If you have an old, maybe broken, "plastic antenna", you can gently crack the plastic away, using pliers, to reveal the half-wave dipole inside.

This is also what you'll find inside (most) transmitters that have the antenna concealed inside the handle - though some of the latest Jumper sets use PCB antennas instead (cheaper, but not as good).

These half-wave dipoles have better gain, and therefore range, compared to the simple quarter-wave whisker alone (which is often used on our receivers). There's no need for it on receivers though -it's heavier, takes up more room, and is a little less omni-directional (it has less gain along the axis of the antenna).

The directionality thing is why you, ideally, shouldn't point your transmitter antenna directly at your plane - you get the best range when the antenna is aligned at right angles to the plane's direction.
HenkB
Posts: 44
Joined: 01 Feb 2019, 11:19
Location: Harderwijk, The Netherlands

Re: Repost: Hidden 2.4ghz transmitter aerials

Post by HenkB »

Thank you guys for the explanation. Very much appreciated.
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