Aerial-to-Aerial Coupling

Finbar O'Connor EI0CF and Alan Melia G3NYK

There has often been talk on the RSGB LF Reflector of the problems of using two aerials on LF. The usual instance is of a 'T' or inverted 'L' being used to transmit and a loop to receive. The idea is to use the loop to null out nearby interference, and leave the wanted signal in the clear. The effect of running a loop close to a tuned 'T' is that there nulls on the loop become 'blurred' and a lot of noise is received on the loop. What seems to be happening is that the currents in the tuned 'T' aerial, due to received signals, are inducing currents into the loop. I suppose the 'T' could be regarded as a single turn loop with the effect of the ground. If this was the case one would expect the coupling to depend on the orientation of the loop. I have never actually been able to confirm this, nevertheless I make use of the effect by allowing the coupling to feed signals to two separate receivers at almost the same strength. It makes an almost lossless diplexer, for use at times when I dont need the directivity of the loop.

When Finbar started to do aerial measurements, he built a version on the ground-loss bridge to help optimise the system at his new QTH, on the shores of Trawbreager Bay near Malin. After some interesting results highlighting the effect of the main supply earth connection (tabulated in the aerial loss bridge page) he errected a couple of 18foot bamboo poles and slung a 100 foot wire between them. The intention was to get some more experience with the bridge and try to understand what it was telling us. The first set of measurements initially perplexed Finbar, but he quickly realised that the main 60 foot vertical was still tuned to 136kHz. The easiest thing to do was de-tune it as it was a trip outside to disconnect it from ground. Sure enough, the effect Finbar was seeing, moved with the retuning, up to about 150kHz. The table of results is given below and clearly shows a severe increase in loss at the resonant frequency of the vertical. The loss resistance climbing to nearly 400 ohm. This can be explained by remembering the operation of a GDO where the power is sucked out of the oscillator by a nearby resonant circuit. The increased resistance is the effect of the coupling to the nearby aerial, in the same way that radiation resistance represents that portion of the signal that is lost to the aerial circuit by radiation. This also enables one to appreciate how the aerial loss resistance increases when power is coupled into nearby 'environment' (trees, buildings, masts). In this case the 'parasitic' is not resonant so the result is not so dramatic. Mike G3XDV has reported incresed aerial current and more stable tuning when he earthed the pole on the end of his house supporting the feed end of his wire. The surmise was that the loss induced by the unearthed metal pole was due to the induced currents having to flow through wet brickwork. It does however indicate that Mike was coupling significant power into the support pole, and one wonders whether a pair of insulating clamps to the T & K wall brackets might not have been an even better solution.

Note the three separate repeated measurments at 136.5kHz as checks. Then when the aerial was detuned....

The vertical was resonant here at around 150kHz, so the wire was disconnected from the (grounded ) coil, then...

It is also interesting to note the closesness to 6pF per metre for the estimate of aerial capacity. The actual capacity measured, is a function of the aerials natural resonant frequency (at a quarter-wavelength). The equivalent circuit is of an inductance, capacitane and resistance in series. One can assume that because the resonant frequency is about 2.3MHz the capacity is close to 215pF, the inductance is that which would resonate with 215pF at 2.3MHz, or about 22uH. As the measurements move closer to the natural resonance the reactance of the inductance rises and the reactance of the capacitance falls. The capacitance measured by the bridge is the difference between the capacitive reactance and the inductive reactance, they have different signs but below resonance the capacitive reactance is the larger. Close to resonance the difference in reactance is quite small, and a small capacitive reactance is a big capacitor. This effect is not so obvious on the above tests as we did not measure close enough to the natural resonance of the wire. The effect is shown better in the table below where the wire was extended to 142 feet (abt 46m) which took the natural resonance down to 1680kHz

Note that the resistance climbs rapidly as the measurement frequency approaches the natural resonance. I had assumed that this was due to the radiation resistance increasing, but the effect seems to be too big for that. I think the maximum radiation resistance of a quarter-wave aerial is about 36 ohm. The loss resistance can seen to be dropping with incresing frequency as shown by Jim's M0BMU series of measurements on his aerial. This is a bad sign for 73khz and shows why it is so hard to radiate a good signal on that frequency.