I've heard from a number of Amateurs on the bands that I will get much better performance if I cut the feeders for my 2m and 70cm beams in odd multiples of a quarter-wave.
Not being someone who believes all that I'm told on face value I thought I'd investigate further.
A quick caveat - I'm ignoring line losses here so remember to take that into account. Ecoflex 10 at 200MHz has a loss of nearly 6dB per 100 metres meaning a quarter of the generated power will be lost.
The RSGB Full Licence Manual states the following regards standing waves - "If the line is terminated in a resistor of value Zo, it's characteristic impedance, the resistor will absorb all the power."
So in which case as long as the load we are transmitting into is of the same impedance as both the generator and feeder we will radiate all the power.
Now to double check lets check what the current ARRL handbook says, section 20.1.2 has the following to say - "In a matched transmission line, energy travels along the line until it reaches the load, where it's completely absorbed (or radiated if the load is an antenna)."
So again this indicates that as long as the transmission line is matched to both the generator and load we will radiate the maximum amount of power, which is exactly what Ohm's law states as well.
Lets triple check just so we know 100% what we have read is correct.
Foundations of Wireless by M. G. Scroggie (1958) Pages 180 says the following - "In accordance to ohms law, the power going into the line during this transient state is a maximum if Zo is equal to the generator and load resistance."
This would indicate that it doesn't make any difference if we have an odd quarter-wave of coax feeding our antennas whatsoever as long as the characteristic impedance equals both the generator and the load.
I think where people are getting this myth from is confusing feeder with quarter-wave transformers used to match unequal loads, where the unequal load does cause an impedance and phase transformation.
Interesting article. I think that the reason people suggest using odd electrical quarterwave multiples of feeder (measured back from the transmitter or RF ground)is to minimise common mode currents. ie currents that can flow on the outside of the coax braid causing it to radiate and potentially pick up noise on receive. Odd quarterwaves will ensure there is a reasonably high CM impedance at the feedpoint. 73's Mike G4BNO
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