20m 1/4 WL Vertical

[Hippo]

I'm in the process of modifying a CB antenna (1/2 WL vertical 'Silver Rod' type) to work on 20m.

I've removed the matching coil at the base, what should be the optimum length?

I'm assuming approx 5.05m for a 1/4 WL vertical at 14.1 Mhz.

I connected an MFJ antenna analyser to the antenna (very poorly sited, horizontal antenna about 2 feet from the ground, indoors at work) and its resonance was around 17 mhz indicating it needed to be longer.

Until I can get in the back garden at the weekend I wont be able to make any better measurements. I am hoping to mount it in the air with 2 or 4 1/4 WL radials.

I would be interested to hear people's thoughts, or if anyone has done this before.

Regards,

Andy
2E0BSC.

[G0BXU]

Andy

If you look 8 topics below this one on the listing page, you'll find a topic titled "turn a silver rod 27mhz antenna into an 80mtr" this might help with the information you need

Alan

[G0GQK]

Looking through my book of words a quarter wave vertical for 14.060 Mhz needs to be approximately 16ft 8 ins long and a radial would be the same length. Shorten it for resonance about 14.200 Mhz

If you consider making a 40 metre vertical the length would be 35 ft 4.5 ins, fed 4 ft above earth and radials of plastic covered wire need to be 33ft 3 ins long. This antenna can be tuned on other bands but the angle of radiation would be high on some bands.

The approximate length of radials is made by the formula 232/F Mhz

G0GQK
A Marconi antenna for 160 with ground radials would be 117 ft

[Hippo]

I put the antenna up today, I didn't have chance to give it a good testing as once I had finished 20m was starting to close.

The base of the vertical is currently about 10-12ft from the ground. I have installed 2 1/4 WL radials. SWR is currently about 1.3:1 according to the MFJ analyzer.

The radials are sloping at approx 30-40 degrees. I cannot have the radials 180 degrees apart, one of them appears to be interacting with my 2m vertical pole which is about 1m away.

The solution at the moment is to have the affected radial at 90 degrees to the other (looking from above).

Will this cause any problems with my radiation pattern?

Early indications are that it performs 2-3 S-points better than my multiband inverted-L.

One wierd thing I noticed playing with the MFJ. It also seems to be resonant on 2m!

[G0GQK]

Pleased to know the antenna is working well. Don't be concerned about where the counterpoise radials are facing it won't make any difference. Having messed about with verticals, the rule about a 45 degree drop never worked for me, I could make the SWR 1:2 when the counterpoise was parallel to the ground. !

The swr of 1:3 is OK, all we need now are some sunspots. The A&K are poor again today. Enjoy yourself, there are plenty of European stations to contact.

Mel G0GQK

[pinkfoot]

Hi Andy..

Probably having the radials sloping as you are is helping the antenna get close to 50 ohms at feedpoint I'd guess..

Pattern wise, it will be VERY slightly directional in the apex of the radials, i.e the narrower side, albeit, so slight that you wouldn't notice it.

If you can get some more radials under it, it will work VERY well, It sounds like you have done your homework and got it right. Elevated radials will always outperform buried ones, it's just having the space to do it!

As for the 2 metre resonance, is it where reactance is at 0 on your MFJ or is it purely that the SWR is low? Two different things altogether. I once had an MQ1 minibeam which besides the 20-6, worked quite well on 2 metres! I even managed to work a station in Devon ( I was near Blackpool at the time).

Best Wishes and enjoy the DX!

Kev
M0TNX

[Hippo]

I thought I would add a couple of pictures to this thread.

The first shows the new antenna next to my 2m/70cm co-linear.

The second is the view of the MFJ when connected to the new antenna.

http://www.prog-tech.co.uk/ham/antennas.jpg

http://www.prog-tech.co.uk/ham/mfj.jpg

[steveeh131047]

Andy,

EZNEC5+ confirms the good VSWR you are seeing for that antenna.

Interestingly the azimuthal pattern distortion is very small at low elevation angles, but becomes quite noticeable at higher angles. The worst case is at a take-off angle of 50 degrees when the eccentricity in azimuth is at least 12dB - the favoured direction is along the line bisecting the two radials.

Slightly off-topic, but I wonder if everyone realises you only need 2 radials (180 degrees apart) for an elevated ground plane to produce an almost omni-directional pattern:

George Brown, the inventor of the ground plane antenna, found that only two radials were necessary. But when his company went to sell it, the marketing department decided that no one would buy a two-radial ground plane antenna in the belief that it would be omnidirectional. So they added two more to make it "look" more omnidirectional. The four-radial ground plane persists to this day.

Don't you just love antennas ?

Steve G3TXQ

[pinkfoot]

Go on Steve... What's the effect of adding more elevated radials?


Kev

[steveeh131047]

Originally posted by pinkfoot
Go on Steve... What's the effect of adding more elevated radials?


Kev

Kev - did you mean on Andy's particular antenna, or generally?

Steve G3TXQ

[pinkfoot]

Generally...

Elevated radials is an area I haven't read too much about, and more importantly had the chance to experiment with.

Seen a Big IR today, and it has me wondering.. Charles OXO elevated radials on his and had great success, I was wondering if the extra space 2 would take up would be worth the real estate loss...

Kev

[steveeh131047]

Kev,

OK, the "small print" first. I modelled a 20m quarter-wave vertical over quarter-wave resonant horizontal radials. Everything was elevated 10ft above "average ground" and the elements were all #12 gauge copper.

This is what I found:

1 Radial: VSWR 1.2:1, Worst-case azimuthal eccentricity 20dB @ 40 degrees, Gain at 10 degrees take-off angle -1.8dBi

2 Radials (180 degrees): VSWR 1.9:1, Worst-case azimuthal eccentricity <1dB at all angles, Gain at 10 degrees take-off angle -1.3dBi

4 Radials (90 degrees): VSWR 2:1, Worst-case azimuthal eccentricity <1dB at all angles, Gain at 10 degrees take-off angle -1.5dBi

The message seems to be that going from 1 radial to 2 is the big step; it raises the VSWR significantly and restores the omni-directionality; thereafter results are indistinguishable.

Angling the radials downwards by about 20-30 degrees restores the VSWR in the case of 2 or more radials.

73,
Steve G3TXQ

[pinkfoot]

Food for thought indeed.. I was always under the impression that even with elevated radials, more was more, if you see what I mean? I understood and had seen the difference with a commercial antenna's VSWR when using 2 and 4 radials, was unsure about what effect it had on gain and eccentricity..

Hmmmm... time I made the centre loaded 80 vertical again, this time with elevated radials...

Thanks Steve and to Andy for posting such an interesting topic

Kev

[steveeh131047]

Originally posted by pinkfoot
Food for thought indeed.. I was always under the impression that even with elevated radials, more was more, if you see what I mean?

Kev - More is certainly better if you are talking about buried radials, or radials very close to the ground. It's less the case with resonant, elevated, radials.

I know it's only one man's experience, but read the following posted by W5UN:

In 2004 I constructed a 1/4 wl 80 meter ground plane antenna, and installed a 3/16 wl radius screen on the ground under it. The screen was made up of a grid of #14 copper wires, with 36 inch spacing, bonded with plumbers solder at each and every grid crossing point. On top of this grid I also installed 32 equally spaced 1/4 wl radials. A second 80 meter ground plane antenna was constructed several hundred feet away (this was to be the first of my 4-square array). This antenna was constructed with the base raised 10 feet off the ground, and 3 elevated radials were installed parallel to the ground so that grazing cattle could move about freely without doing damage. The radials were made about 10 percent shorter then a quarter wavelength, tied together at the base of the vertical, and the antenna was then brought to resonance by placing a single coil between the radials and the coax shield connect point. I was able to select either of the verticals with an A-B switch at the operating position. I ran this system for an entire season, running comparisons on both receive and transmit, and relying on DX stations to give me comparative readings when I would switch antennas. My experience is this: On receive I could tell very little difference (if any) between the antennas. On transmit, I would often get better reports on the elevated ground plane. This was very surprising to me, as I had always heard that more radials make a better (read better for DX) antenna, and that the elevated system should produce a signal strength of up to 6 db lower then a well constructed radial system would have. I never got a better report on the ground mounted antenna, but on a lot of occasions the listening stations would say the signal strengths of both antennas were identical. My conclusion was that I should go with the elevated radial system.

73,
Steve G3TXQ