Passive Antennas

Passive Antennas and Reflectors can either be a savior or a curse. In Sydney many people have traditionally had ghost on their TV picture thanks to signals reflecting from the Sydney Harbour Bridge. The huge metal beams create a huge surface for signals to bounce off. Many other cities have similar problems.

From the ARRL Microwave and UHF Experimenters Manual:-

P r =

PtGt Grλ2S

+
4 πRt2 (4 π)2Rr2
where

P t = Transmitted Power – Watts
P r = Recieved Power – Watts
G t = Transmitted Antenna Gain – As ratio
G r = Receive Antenna Gain – As ratio
R t = Range from Transmitter to Reflector – Meters
R r = Range from Reflector to Receiver – Meters
λ = Wavelength – Meters
S = Cross section area of reflector

However this is for reflections off a solid object. We can also simplify this in terms of path loss the following way.

Lr = 10 Log

(

λ 2 S

R t2 R r2

)

– 153

If we look at that equation, and ignore S we see that the loss is minimised when one of the distances in minimised and the other is maximised. The reason for this is that this is the case where most of the transmitted power is reflected.

Another way to think about this – when you have have a couple of antennas connected back to back running as a passive repeater. The passive antenna will only re-transmit the energy that it picks up.

The value for S in the case of two antennas back to back is beyond the scope of this WWW page. Other common devices for the reflector that is S are water towers and steel bridges.

OK. The question becomes what affects S. It is dependant on a number of factors. These include

  • Capture Area of the antennas
  • Directivity of the antennas
  • Efficiency of the antennas
  • Feedline losses

Some of these factors are actually used in the calculation for the GAIN of the antennas. I suspect that S can be computed if you take the ANTILOG of the antenna gains added, with some constants and multipliers. I think it might ge something like S = k log -1((Gain 1Gain2) +n), but that is a guess.

How to build a Passive Antenna

A passive antenna for 802.11 is quite simple. All you need to do is connect two normal antennas together by a SHORT piece of cable. Both antennas and the cable should be 50 Ohms. It is important to get as much gain on both back to back antennas as possible.

It is important there there is NO AMPLIFIER between the antennas. 802.11is bi-directional. Amplifiers generally only amplify in one direction leading to problems.