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Author Topic: Dipole vs. Vertical  (Read 3048 times)
STAYVERTICAL
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Posts: 875




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« Reply #15 on: November 26, 2012, 01:02:27 PM »

All antennas are compromises. As many have noted - there is no single antenna that is best for all locales, band conditions, and operating styles. However - if you only have room for one HF antenna & don't mind putting down a good low-impedance radial field, and if you have average or better ground conductivity out to 100 WL, and want to work the most directions with a low-angle signal, but still have enough high-angle radiation on the low bands for NVIS - a remotely-tuned inverted-L is a good choice. Note that there are a lot of 'ifs'. That's simply the way it is with HF antennas. Of course, the compromise for all of that horizontal coverage & high-angle NVIS is lower overall gain than say, a high-mounted remotely-tuned doublet or horizontal loop. However, gain alone is often less important than putting your signal where there is a high likelihood of it being received. And that is where a properly-designed, remote-tuned, multi-band inverted-L shines. See L. B. Cebik's excellent info on antennas for details:

http://www.users.on.net/~bcr/files/backyard%20wire%20antennaes.pdf

http://www.antennex.com/Sshack/books.htm

I agree with you there.

One of the best multi-purpose/multi-band antennas I have ever used was the old style single wire fed windom.
When paired with one or more counterpoise wires (only a few feet above ground), it performs well on both DX and local contacts.
I guess it is really a massively top loaded vertical which goes dipole mode at certain frequencies, but it is certainly a good overall performer.
The inverted L you describe seems like a variant of this, so I am not surprised it is a performer as well.

Because of the long top section you don't get the overhead null of a standalone vertical so locals also get a chance.
As you said, the secret is the remote atu at the base of the vertical section.
Where the windom is perverted these days is in making it into some sort of g5rv or OCF dipole, by feeding it with coax right to the horizontal wire.
In that configuration you don't get the benefit of the vertical wire section, unless you have common mode currents, which is undesirable anyway.

So a classic single wire fed windom (or an inverted L), with a remote ATU, will give you dx and local contacts.
Although I use a vertical for DX personally (QTH restrictions), my experiments with the classic windom left me very impressed with its good overall performance.

If there is one thing I can recommend to antenna restricted or experimentally minded folks, it is get yourself a remote antenna tuner.
Your universe will expand enormously, and your frustration level will drop exponentially.
Much of the average hams time is spent trying to match an antenna's impedance to 50 ohm feedline, rather than experiment with antenna's.
With a remote ATU you just press a button and the matching is done.
Then you can concentrate on seeing how efficient it is and what its pattern looks like.

Another simple but extremely useful accessory for antenna experimenters is a field strength meter.
It does not have to be expensive - you can use two 1N34 diodes one capacitor, one resistor,  and a digital volt meter if you wish.
Alternatively, many of the cheap SWR meters have a FS (field strength) facility built in where you attach a whip or wire loop.
(Obviously it is taken out of line in field strength measuring mode).
This will save you having to build anything - and they abound at flea markets for a few dollars.

Amazingly, these cheap meters work up to at least 2 metres (maybe more but I have not tried that).
Many suggest using a whip, but I have found a loop of wire around 1 foot diameter is more stable in pickup mode.
Just take it from the FS terminal to the case of the SWR meter.

Rather than obsess about swr and wonder how the antenna is getting out - use your simple field strength meter to see the effect of any changes.
Now, the antenna pattern is 3 dimensional, and not many of us have pixie wings to fly around the antenna with a field strength meter.
But you can at least measure the low angle radiation strength and see what effect your changes make.

One thing to keep in mind when doing antenna measurements is the difference between near and far field radiation.
Basically in the near field (around one wavelength from the antenna) the electric and magnetic fields are still organising themselves into an electromagnetic wave.
They exist as separate entities and interact with each other to produce the travelling, never to return electromagnetic wave of the far field.
There is a complex dance in the near field region, with energy being provided and returned, so impedance of the antenna is influenced by what you do within the near field region.

This is why adjusting a beam antenna's parasitic elements influences impedance at the drive point - they are in the near field.
Once the energy is in the far field region - they have basically left the near field nest.
Their ability to influence the feedpoint impedance is very minimal in the far field region.
At this point they are a fully grown up electromagnetic radio wave.

This is why many people advise taking field strength measurements at sufficient distance to ensure you are in the far field.
Unfortunately, many of us living on small suburban plots would find this very difficult.
So my advice is - any measurements of field strength are better than none.
And in my experience, as long as you have a path clear of distorting metal structures - the measurements of near field strength are fairly indicative of performance.
Of course get the most distance you can - but don't avoid taking field strength measurements just because you can't get 1 wavelength distance.

The point of the above novella is to urge antenna experimenters to consider field strength measurements as an even more important part of antenna assessment than SWR.
SWR matching is nothing more than impedance matching from the antenna impedance to 50 ohm feedline in most cases.
It does not affect antenna pattern or antenna efficiency at all - it is a really trivial part of the antenna equation.
SWR only affects feedline loss - end of story.

So get those cheap CB SWR meters out of the closet and use their field strength function - it will lift the cloud of uncertainty around how your antenna is really radiating.

73 - Rob
« Last Edit: November 26, 2012, 01:11:23 PM by STAYVERTICAL » Logged
N4JTE
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Posts: 1158




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« Reply #16 on: November 26, 2012, 03:13:04 PM »

Rob that is one of the best explanations of verticals I have read anywhere!
Good solid information.
Bob
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