HF Antennas

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Re: HF Antennas

by G0KZZ » Fri May 13, 2022 6:28 am

VK5TM wrote: Sat Nov 30, 2019 11:54 pm I think you need to get rid of one anaylser :lol: (Old Chinese proverb:- Man with two watches not sure what time it is. Translated into hamspeak - Man with too much test equipment mighty confused :o ).
If the equipment is calibrated correctly there shouldn't be any issues or confusion, if anything it would reinforce the belief in the accuracy of any results obtained.

I have a number of assorted antenna analyzers here, ranging from YouKits ones, MFJ, and Kuranishi, and they will all give pretty much the same indications (within a gnats knacker to use an expression), when connected to the same load to be tested. Same with the minVNA too.

What tends to cause wide variations in readings between units (apart from the obvious calibration differences), is when the person performing the test/s forgets that the outer shield of the coaxial feeder may be 'hot', or in the case of balanced feeder the fact that as soon as it is directly wired to the unbalanced input of an antenna analyzer (pretty well all of the Amateur ones use unbalanced connections e.g. SO239, BNC, N-Type etc.), the 'balanced' feeder has then effectively been grounded on one side, and in the process is no longer balanced.

Keeping the above in mind, you will then get the situation where the analyzer/operator now forms part of the antenna circuit themselves! This can often be detected by noting the readings of an analyzer while being held by the person performing the tests, and then placing the instrument down on some insulated surface and observing any changes. In a lot of cases simply moving your hand near to and then away from the case of an analyzer will cause noticeable changes in SWR and impedance readings, which in turn shows that you are not seeing just the characteristics of the antenna/feeder combo, but also the unwanted stray 'loads'.

Hope that makes sense!

73, Mark...

Re: HF Antennas

by G0DJA » Sun Dec 01, 2019 12:14 pm

VK5TM wrote: Sat Nov 30, 2019 11:54 pm (Old Chinese proverb:- Man with two watches not sure what time it is. Translated into hamspeak - Man with too much test equipment mighty confused :o ).
Yes, LOL.

At least a watch that has stopped is right twice a day, as long as it's analogue, of course...

Re: HF Antennas

by VK5TM » Sat Nov 30, 2019 11:54 pm

I think you need to get rid of one anaylser :lol: (Old Chinese proverb:- Man with two watches not sure what time it is. Translated into hamspeak - Man with too much test equipment mighty confused :o ).

Re: HF Antennas

by G0DJA » Sat Nov 30, 2019 3:41 pm

One analyzer said that the 40M antenna was not working, another said it was OK...

I think it needs new batteries in the other analyzer.

Re: HF Antennas

by G0DJA » Sat Nov 30, 2019 1:48 pm

Thanks Mark,

I tried that antenna arrangement when I lived near Wakefield and struggled a bit with it. I couldn't get the claimed resonance on various bands. In the end I put it down to the feeder being too close to the house.

I've got a dipole for the 40M band up at the moment, but not given it a try yet. Maybe today will be good as it's very foggy so I wont be tempted to get the motorbikes out...

Re: HF Antennas

by M0VMT » Wed Nov 20, 2019 10:58 pm

Dave take a peek at the zs6bkw plenty on the web but here is a link
http://www.sarc.org.au/wp-content/uploa ... VK2ACD.pdf

Re: HF Antennas

by G0DJA » Mon Nov 04, 2019 7:52 pm

This is the area that I have to play with.

The house is a typical two storey semi with a pitched roof. The pole holding the 23cm Yagi/Uda array is in the middle of the house line above the garage (The ridge runs left to right) and, therefore, not in the middle of the plot.

The plan is not to scale. I ran out of page when plotting the front garden/driveway.
Attachments
IMG_20191104_0001.jpg

Re: HF Antennas

by G0DJA » Mon Nov 04, 2019 7:15 pm

I got the ladders out earlier and had a look at the existing antenna.

The 'temporary' fixes, using chocolate block connectors on the wires had corroded and the ladder line feed snapped off when I was trying to see how the centre connections were looking. So, I replaced the wire with another dipole for 40M and a coaxial cable feed instead of the open wire.

Looking at the antenna system with an antenna analyser shows that the new system is resonant on 40M, which at least gives me one band to play with.

I'll try and put up a dipole for 80M as well, although the natural length of my gardens (front plus back) seems to suggest that 60M might be more appropriate. I kept my NoV for that band up to date, so might have a go on that band for a while again.

HF Antennas

by G0DJA » Mon Nov 04, 2019 1:36 pm

I've been measuring up the available area for HF antennas and I reckon that I can just about fit a half wave dipole for 80M in, as long as I bend the ends a bit. The height above ground is only going to be about 10M at the centre point though, so likely that it will be best for NVIS propagation.

I might parallel it with the existing doublet and see how well it tunes up with the 'Z' match.

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