Posted by
Vic K2VCO on
Dec 20, 2010; 5:46pm
URL: http://elecraft.85.s1.nabble.com/Inverted-L-was-OT-Vertical-antenna-tp5852957p5853227.html
Your post is a little elliptical, but am I to understand that you are saying that there
are resonant effects noted with in-ground radials when the wire is insulated, but not when
it's bare?
If that's true it's interesting and I hadn't heard it before. Most writers on the subject
simply say that if the wires are in or on the ground to ignore the question of resonance
and just make them as long as possible -- and they don't mention the use of insulated or
bare wire.
I have always thought that the connection to ground from an in- or on- ground radial
system was primarily capacitive and that insulation or lack thereof was irrelevant.
I'm not challenging what you say, just interested in the idea. I've always used insulated
wire just to improve resistance to corrosion.
On 12/20/2010 9:28 AM, Guy Olinger K2AV wrote:
> (For the record, I am also addressing some off-reflector extensions of
> this thread with a single post.)
>
> We are still talking about an end-fed antenna for 80-10 which presents
> unique problems. Trimming the horizontal length of up 30, out 30 for
> a good match will help a lot. But that will not address avoiding an
> up to 10 dB loss problem in the radial system that can make it perform
> like a wet noodle dipole.
>
> Although there really is not that much published on this, LOCAL
> research, I repeat, RESEARCH, does indicate a lossy booby trap in what
> is done at the base with radials. Adding the high bands to the radial
> calculations takes away from using any insulated radials because the
> radials (MEASURED, not modeled) can have velocity factors as low as 45
> percent and as high as 80 percent laid on or notched into the ground.
> A range of 54 to 76 was measured in a single back yard, just moving
> the site and orientation of the measurement. Picking and keeping an
> anti-resonant length for insulated radials, as in the vertical length,
> is an impossibility.
>
> If any of you want to prove this yourself, put down a 151 foot (46
> meter) dipole on the ground (DOG). Insulate the ends so they don't
> short to ground and measure the resonance point and feed resistance.
> Use the handbook formulas to compute velocity factor. Scan across 160
> through 40 meters and make a graph of the varying readings. Let it
> lay on top of the grass, measure it, notch it in and re-measure.
> Measure in wet weather, and measure after things have dried out.
> Measure it in different places in your yard, note how it changes. Post
> your measurements as you go. Put down a 151' BARE WIRE dipole and lay
> it on top of the grass. Measure. Water it with a garden hose and
> remeasure. Now notch it into the ground (needs to be in contact with
> the dirt all the way) and remeasure. In particular note how the feed
> Z measurement levels out over frequency, how the Z remains more or
> less constant across the frequency range. This exercise will help you
> in thinking about getting on 160 meters. It should also make you
> really suspicious of insulated radial claims for multiband
> applications.
>
> I have a list of call signs who are dismissive of these radial
> concerns AND ALSO argue that 0.3 dB is significant on RX. This
> particular schizophrenia is really hard to understand. I understand
> the reverse, someone who considers 0.3 dB significant being a really
> snotty radial purist. He's sweating the little dB parts any place he
> can dig them up.
>
> Don't wonder if this is truthful or not, or worry that it's not the
> common wisdom (whatever that is). Just go out and measure it
> yourself, and spend a contemplative cup of coffee at a quiet time
> about how YOUR measured results will effect use of radial wires on/in
> the ground. In analyzing this, remember that this scales to 30 feet on
> 10 meters quite nicely, that the competition is multi-element yagis,
> and if anything, ground losses on 10 meters are MORE significant than
> low bands where the enemy has more of the same constraints.
>
> Now contemplate how the ENDS of insulated wires are going to corrode
> and arc through (that's a voltage point at the wire ends) over time
> and change the radial system behavior over time.
>
> You could prove in this bad behavior on 160 (where radials are common
> and an everyday issue) if you want by putting down 120 insulated 250'
> radials and do a time study. You don't hear about this, proving it's a
> disaster, because people want to spend their precious constructing
> time and even scarcer construction dollars on something that will
> work, not disproving an idea that won't work. That's NOT a dig at
> anyone, radial construction time and money when you're raising a
> family and/or building a career IS precious.
>
> Insulated radials are subject to resonance effects which depend on
> today's ground moisture to set today's velocity factor over/in your
> particular dirt and complicate the performance of the radials in the
> same way that the length of the vertical complicates selecting lengths
> there. The difference is that you get it set for the vertical wire
> and you are done. This becomes a ridiculous moving target for
> choosing a decent multi-band length with an insulated wire on the
> ground.
>
> If you're looking for some validation in the commercial world, the
> simple commercial reality is that most people simply do not want to
> deal with radials, will short-cut the procedure and then blame the
> awful results on the antenna itself. Radials are a total commercial
> nightmare for the ham market. All the commercial manufacturers avoid
> this conundrum by using some counterpoise technique on the high bands.
>
> If the radials are not dense, the vertical wire will get the blame. It
> will get the blame regardless of whether it is straight up or an L.
> But the 1000 pound gorilla in the room was always the radials. I do
> not know why hamdom seems so oddly dismissive of this. Putting
> radials down IS a real PITA. But 60 bare wire radials buried just
> under the sod are completely invisible from the start, and lack of
> this commercial FCC grade treatment of the problem is why so many
> installations of an otherwise great stealth antenna fail.
>
> When you are stealth, you have to squeeze every last dB drop of blood
> out of that ground turnip.
>
> 73, Guy.
--
Vic, K2VCO
Fresno CA
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