View Full Version : Radio interference and aerial position

Wally Hayward
7th June 2010, 08:28
Posting in another thread reminded me that someone had asked about the Shadow aerial position (can't find it at present)

A "strange" effect I experienced when I first got my Shadow, was that on the ground, taxying, or on finals the radio was fine. Transmit strength 5 readability 5.
In the air transmissions became unreadable by others and reception became much poorer with lots of interference..
VSWR was fine on the gound.
The problem was only found by flying with the VSWR fitted when one could see the vswr increase well out of bounds at speed.
The previous owner had fitted a whip aerial on the cockpit roof, just in front of the wing D box.

Over time this had weakened such that the whip was contacting the D box in flight causing the problem..

8th June 2010, 13:08
It might have been me. Due to RF interference from an LCD panel I was asking about the tail mounting - In the end I moved my antenna from the cockpit position to the tail. I found that the RF (thankfully) was reduced and also reception was better.



18th June 2010, 07:57

I was wondering if my radio problem is common. My aerial is above the cockpit in front of the D box. When I am fly towards or at 90 degrees to ATC the reception is excellent with fantastic range, but flying away from ATC they can't hear me at all (All that wing/fuselage in the way). Would moving the aerial to the tail just swap the problem or is it a better position all round?


Wally Hayward
18th June 2010, 08:30
I can only speak from my situation.
With the aerial on the cockpit in front of the D box I could never get a better VSWR than about 2.5:1, (meaning only a small percenntage of the power was actually being radiated) no matter what ground plane set up I used. I did not experience your problem, but overall it was not very successful, particularly as mentioned, in flight.

Certainly the painted ply D box in my case was also influencing the radiation pattern on a field strenght meter.

Mounting on the boom just above the fin, which is actually the factory fitted position, I instantly got a VSWR of around 1:1 over the band .
Also the small amount of recieved ignition interference experienced with the"cockpit" position was non existant with it over the "tail"
If you do change positions, make certain you use 50 ohm RG5/U or better coax and 50 ohm BNC connectors (75 ohm BNC look very similar).

Maybe worth asking if someone has a VSWR meter (which will cover the airband) you could borrow to check.

18th June 2010, 10:17
Many thanks Wally,

I will have a look at moving the aerial to above the fin (I wonder if anyone has a picture of the fitting?), I will also check the cable and connectors.


18th June 2010, 11:04

When I moved mine, I did start to get some engine interference, which was probably due to the wiring. I fitted a capacitor across the power supply to the intercom (as the radio is fed from the intercom) - which got rid of most of it instantly. Cost about a 5, easiest fix I've ever had!

Got it from partsforaircraft.



Wally Hayward
18th June 2010, 11:30
The position of mine is attched.
I think it probably is the same as the factory position in that, although the aircraft had a cockpit mounted aerial when I got it, the original mount was left in the boom.

Attached files http://cloudniner.co.uk/images/converted_files/81477=6184-aerial.jpg (http://cloudniner.co.uk/images/converted_files/81477=6184-aerial.jpg)

19th June 2010, 13:27
Before I moved mine, I checked with Fiona from SFC, who said that the one they had was 9cm aft of the fin post (centre to centre) - I did check pictures of others and this seemed to coinside too - so this is where I put it.



Wally Hayward
19th June 2010, 16:44
Just a point, but bearing in mind that the Shadow was designed, produced and flown in the days before the current, to my mind overindulgent and obsessive, adherence to the theroretically implied spectre of Setion S, then I suspect that no one anywhere has ever done any calculations, or tests, as to the stress, etc, implecations of fitting the aerial to the boom. Hence the variation in quoted position of 9cm and for example mine at 25cm.

RF wise there isunlikely to be any difference within these limits... so "yer paes yer muney and makes yer choice"

23rd June 2010, 16:23
I think you are totally right, it's just a case if someone gets caught up in the god awful mod process. I have bitter experience of this recently. I've seen numerous customisations on shadows and others - most of which are totally sensible. I saw a shadow with an inclined radiator setup, so there was loads of free space around the prop and less drag. Given this unusual mounting I hate to think what hoops needed to have been jumped thru - assuming it had. ;-)
It may have been easier a few years back, certainly the cover-your-arse approach now makes it a lot more difficult to make even the simplest of improvements.