RF field strength near wire fences and other conducting objects

[Sueno]

In our aeromodelling club we sometimes (even often) have crashes on the approach to land. Some members blame poor radio installation, some poor construction, others RF interference, some pilot error (very few).

The cross-wind and downwind legs pass over a long three strand wire fence at low altitude. The strands are not connected to each other except by wooden posts, which are often wet. The soil is acid. The frequencies in use are 35MHz and 2.4GHz.

Is it possible, that the fence acts as a sink for the RF field and low field strength in the "RF shadow" of the fence. Would earth bonding the wires together and connecting to grounded copper conductors help or worsen any such effect?

 

[JimB]

OK, second attempt at a reply.
The first one got lost who knows where,either my PC or the board had a bit of a fit.

In our aeromodelling club we sometimes (even often) have crashes on the approach to land. Some members blame poor radio installation, some poor construction, others RF interference, some pilot error (very few).

Are the crashed equally distributed between 35Mhz and 2.4Ghz controlled aircraft?
Is there a possibility of some person with mischievous intent deliberately interfering?
Radio antennas are not completely omnidirectional (usually only in one plane) and have deep nulls in their response both on transmit and receive.
A straight wire or rod antenna will have a null off each end, so if the guy with the controller is pointing the antenna at the aircraft, he is transmitting minimum signal in that direction which could of course add to the problem.

The cross-wind and downwind legs pass over a long three strand wire fence at low altitude. The strands are not connected to each other except by wooden posts, which are often wet. The soil is acid. The frequencies in use are 35MHz and 2.4GHz.

I assume this is not an electric fence.
The high voltage pulses can create wideband interference which could affect 35Mz, but unlikely to affect 2.4Ghz.

Is it possible, that the fence acts as a sink for the RF field and low field strength in the "RF shadow" of the fence.

Theoretically yes it is possible, but in practice I doubt that you would notice the difference unless you were very close (less than 2 metres) to the fence.

Would earth bonding the wires together and connecting to grounded copper conductors help or worsen any such effect?

If there is some effect created by the fence (more likely at 35Mhz than 2.4Ghz this would change the effect, but not in any easily predictable way.


A wild idea, there is something called the "rusty bolt effect" it can cause problems on sites where many transmitters are operating simultaneously.
If two pieces of metal are making poor contact (ie they are corroded at the point of contact), a diode can be created.
Diodes are used as mixers and frequency multipliers, so if there are RF currents flowing through the diode junction (rusty bolt), other frequencies will be generated.
Harmonics, if the original frequency is F1, there will be signals generated at 2xF1, 3xF1, 4xF1 etc.
Intermodulation, two original frequencies F1 and F2 can create new signals at 2xF1-F2, 2xF2-F1, 3xF1-2xF2, 3xF2-2xF1.
Do this calculation with some of your 35Mhz channel frequencies and see what happens, you will get spurious signals on other channels.
Has anyone ever tried listening to an affected channel with a radio receiver to see (hear?) if there are any spurious signals being generated?

JimB

 

[Sceadwian]

How are the radios oriented at the time of crash? One thing that most RC'ers don't know is the pointing the antenna directly at the craft is horribly bad because that's where the field of a whip antenna is weakest (regardless of frequency) The antenna should always be at as close to a 90 degree angle relative to the aircraft at all times. The problem with getting shot down like that is it almost never happens the same way twice and there are so many possible causes that it's nearly impossible to track down what's causing it if it doesn't happen 100% of the time. There could be pager towers nearby, cell phone towers, someone could have a leaky microwave oven, powerline surges, any number of a million other possible occurrences, heck even a passing car with bad wheel bearings could shoot you down if the conditions were right.