Big and Small Antennas at Puckeridge]

[Andre' Kesteloot]

James Moritz wrote:

> Dear LF Group,
>
> Here is the report on the "Big and Small" antenna experiments I did at Puckeridge last weekend, and which several subscribers to the reflector contributed signal reports to. Briefly, it appears that the small antenna was more efficient than was predicted from theory. Any constructive comments would be welcome.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
> _______________________________________________
> Big and Small Antenna Experiments at Puckeridge Decca Site
>
> The main objective of the experiments was to compare the received signals from two very different antennas, each radiating the same power, as calculated with the usual techniques used by LF amateurs. The Puckeridge site offered a unique opportunity to do this, since as well as the existing big 99m (325ft) beacon antenna, which has been described before, it was
> possible to put up a small, amateur-style inverted 'L' antenna.
>
> The vertical part of the L was 9m high and about 130m to the south of the main mast. The single 24/0.2 top wire (about 1mm diameter) ran directly away from the main mast, and was 42m long. The whole of the small antenna was off the main ground system, which extended over a circular area for about 120m from the base of the mast. The small antenna ground system had 4 ground rods, 1m long, spaced a couple of meters around the antenna tuner. The fencing wires that ran under the antenna, about 50m long, were also bonded to the small antenna ground, and were not connected to any other fences.
>
> Both antennas shared a near-ideal location on top of a small hill, surrounded by quite flat agricultural land with open fields. At the same time, the antennas were far enough apart to have only loose coupling.
>
> The small antenna was remotely tuned by a motor-driven variometer. the big antenna's matching circuit was fed through what was effectively a power attenuator, which could be adjusted for the same radiated power as the small antenna. Either antenna could be selected using a change-over switch. To minimise coupling, the big antenna was grounded by another contact on the switch when the small antenna was selected. Also, the long leads to the small antenna had common-mode ferrite chokes, to prevent earth currents flowing through them to the main antenna ground.
>
> I set up the system by monitoring the current (Iant) in both antennas. This eliminated the need to take into account losses in the matching networks and cables, which were significant. To calculate the radiated power (=Iant^2*Rrad), the following antenna currents and radiation resistances (Rrad) were used:
>
> Inverted L - Rrad = 0.020 ohms,
> Iant = 2.1A,
> Radiated power = 88mW
>
> 99m Vertical - Rrad = 0.65ohms,
> Iant = 0.36A,
> Radiated power = 84mW
>
> The radiation resistance estimates were based on the simple formulae (see, for example, ON7YD's web pages), EZNEC simulations, and for the big antenna, calculations based on the quoted efficiency figures. These were all roughly in agreement. Exactly equal radiated power was not possible, because the attenuation could only be varied in steps, but the difference is only 0.2dB. Generating these currents required about 240W into the small antenna, but only 0.7W into the big antenna.
>
> The radiated powers are "EMRP", ie. effective monopole radiated power, assuming that both antennas have the radiation pattern of a theoretical short, vertical monopole. The ERP (ie. effective radiated power relative to an ideal dipole antenna) can be calculated by adding 2.6dB to the EMRP or multiplying by 1.8. The apparent increase in power is because of the presence of a ground plane, resulting in the monopole having greater directive gain. ERP appears to be the quantity specified by the BR68 booklet, containing the regulations for UK amateurs. The corresponding ERP's are 159 and 151mW respectively
>
> Using this set-up, I received 12 comparative reports during QSO's, plus 2 listenter reports, and I also did 2 sets of short range field strength measurements. Out of the total of 16 results, one report was that the big antenna was 4dB up on the small antenna, one indicated no difference. The remaining 14 indicated that the signal from the small antenna was on average 4dB stronger (between 3dB and 6dB or 1 's' point). The most likely cause of the 2 divergent results is, in my view, operator error - for example, I might have sent 'big' when I meant 'small', or forgotten to operate the switch. Of course, other sources of error also exist. The reports were received over a wide range of distance and direction, so the 4dB advantage of the small antenna under these conditions seems clear.
>
> With a couple of stations, I also tried the effect of having the big antenna in a resonant condition while transmitting from the small antenna. They reported the difference was no more than 1dB. The antenna current flowing in the big antenna under these conditions was less than 0.1A, so the re- radiated power must have been only a few mW. With the big antenna grounded, there was no visible deflection of it's ammeter due to power from the small antenna. This indicates that coupling between the antennas was not a significant factor.
>
> On the receive side, There was little to choose between the antennas. With the transmit attenuator in place, signals from the big antenna were aproximately 3dB down on the small antenna. The signal-to-noise ratio was practically the same. Flicking between one antenna and the other, it was hard to tell the difference. If anything, the small antenna was a very slight improvement on some signals.
>
> How might the 4dB difference in radiated power between the two antennas arise? It could be put down to a relative error in current measurement of about 60%, but since I used the same instrumentation to calibrate both ammeters, this is extremely unlikely. I believe this source of error is not likely to be more than 1dB. The other possibility is that the values of radiation resistance are incorrect, either the inverted L having greater than 0.02 ohms, or the vertical having less than 0.65 ohms, or some combination of both these changes.
>
> In order to experimentally check the values of ERP and radiation resistance, field strength measurements are required. Knowing the field strength and distance enables the actual ERP to be calculated. I measured the field strength using a Rycom 3136 selective voltmeter and home-brew ferrite rod antenna, which had previously been calibrated against commercial field strength measuring equipment. Two sets of measurements were made at two sites, one 4.1km north of the main antenna, the other 2.5km to the south. ERP was calculated using the formula:
>
> P = (E*r/7)^2, where E is field strength in mV/m,
> r is distance in km
>
> The two results for each antenna were within 1dB of each other. The average result for the big antenna was 85mW ERP. This is 2.6dB below the calculated value. The average result for the small antenna was 230mW ERP, 1.5dB greater than the calculated value. The measurements give an ERP not very far from the theoretical value for both antennas; however, the field strength measurements confirm the small antenna generates about 4dB greater radiated signal.
>
> The conclusions from these experiments are:
>
> -The 99m Decca mast is a much more efficient antenna than the 9m inverted L (about 13% as against 0.08%) at 136kHz.
>
> -The comparative reports on the whole show that the small antenna gave a signal about 4dB stronger than the big antenna, irrespective of distance or direction. No noticeable directional effects occured. This agrees with the theory, which says both antennas should have the same radiation pattern.
>
> -The field strength measurements showed that the 9m inverted L generated 1.6dB more radiated power as predicted by theory, and the 99m vertical 2.5dB less. It should be remembered that +/-2dB would be considered 'good' accuracy for field strength measurements, and so both these results are in reasonable agreement with theory. However, the field strength measurements confirm that the small antenna radiated about 4dB more power than the big one.
>
> The implication for LF amateurs are that:
>
> -Provided you maintain ERP at the same level, there is no advantage or disadvantage to a small vertical antenna over a big one for LF transmitting - both antennas have the same radiation pattern. Of course, this requires greatly increased transmitter power for the small antenna to acheive the same result, which might well be considered a disadvantage.
>
> -There is no disadvantage in using a small antenna for receive - assuming the local noise level is acceptable, and the receiver sensitivity is adequate. At Puckeridge, the limiting factor on receive sensitivity was the Loran sidebands or QRN, depending on the time of day, which is the same as the situation at my home QTH.
>
> Finally, two interesting issues arise:
> 1) Why does the small antenna apparently radiate more efficiently than it should?
> 2) Why do similar small antennas (eg. the one at my home QTH, and G3XDV's antenna) radiate fields several decibels lower than expected from theory? (The "missing decibels" detected in previous field strength experiments).
>
> The 9m high, 42m long inverted L will just about fit in my garden - over the next couple of weeks, I plan to put it up there, and do some more field strength measurements. This will give a comparison between the same antenna in different environments. Watch this space...
>
> ________________________________________________