Friday, 28 October 2011

Resonant Vs non-resonant Dipole

Due to various reasons, restricted space and laziness I had no proper antenna available for 10M. Until now, I have used my 40M dipole and used the ATU to match the horribly mismatched antenna to the transceiver.

Today I made a very simple half wave, rotatable dipole from aluminum tubing and mounted it on a mast 7 metres high (21 Ft). This is only 3 metres (9 Ft) above the roof of the house. The dipole is directly fed with some old RG-213 coax I had at my junk box.

The difference in performance is huge. On average the signals are 4 S-points higher during fast A/B switching between the old and the new antenna. This translates to a 36 dB improvement. Even more convincing proof is achieved, by monitoring the Reverse beacon network. I could only manage to get spotted on the old antenna if I used power well in excess of 100 watts. I am now able to generate a few spot’s whilst running QRP (5 watts).

Unfortunately, with this improved performance of the antenna, the noise floor has also increased. I suspect I will get a better signal to noise ratio, if and when I raise the antenna a bit higher than what it is at the moment.

I am now finally convinced that it is essential to use resonant antennas. Using the ATU to force a match to a non resonant antenna is not very effective and should be avoided.



The screen shot below clearly shows the difference in performance. The spot made by VK6IA and PJ2T were achieved with 5 watts, all the remaining spots are done with 100 watts and the new dipole. I am not able to get spotted at all with 100 watts and my old non resonant antenna.

Now imagine, if only, I had a multi element Yagi up at 100 Ft…….

73 de Pierre ZS6A






3 comments:

  1. Is your 40m dipole fed with coaxial cable or open wire type?

    I'm asking because my main antenna is an 80m dipole that I use on all bands - it's fed with 450-ohm ladder line and seems to perform well 10-80.

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  2. Hi John,

    I was indeed using coax and no doubt that is why the losses are so very high. In fact I am ashamed to admit, but I am using RG-58 so it can’t get much worse Hi….

    I agree that using low loss balanced feed line; the situation may be very different, since the losses will be much less and therefore might be more acceptable.

    73, Pierre ZS6A

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  3. Same effect here .

    I used my 80/40mtr dipoles on all the bands and also thought " hey its working , i am making LOTS of contacts , it must be working very good"

    Until well , i put up a 10 mt vertical on the roof and suddenly signals was 3-4 S-points better , i could hear better and could make much more QSO's without continuous repeat of my callsign over and over and over.

    Since i work for SANDF i have now done the same to all their HF systems in the field,and in bases . Get them to cut dipoles that is resonant for the frequency , add a few reflectors blow them ( They also called them NVIS antennas now ) cos i explained the idea of short range HF comms.

    They also reported much better signals and clearer comms to all the stations in the field.

    So yes the 80-10 Looks like it working but NOTHING beats a resonant antenna.

    Did same test on my 4x4. A 108 whip compared to a Kenwood MA-5 . U cannot even compare the 108 whip. Its VERY VERY poor on reception , with or without a tuner. Chuck the tuner out and using resonant antennas ONLY.

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