Posted by
Guy, K2AV on
Dec 20, 2010; 5:28pm
URL: http://elecraft.85.s1.nabble.com/Inverted-L-was-OT-Vertical-antenna-tp5852957p5853186.html
(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.
On Mon, Dec 20, 2010 at 10:45 AM, <
[hidden email]> wrote:
> Going from loop to vertical involves two very different lob patterns. It
> was never mentioned if the antenna performance was to favor local or DX. Here
> is one which it does not matter as it adapts for both.
>
> A nice compromise antenna which takes into account:
> ---high and low angle lobe patterns
> ---good match (resonant on 80m)
> ---small footprint
>
> ======DESCRIPTION===============================
>
>
>
> INVERTED-L: 33 ft vertical, 33 ft top wire, feed at base against 12 16ft
> radials. I use Radio Shack low loss 300 ohm tv wire for feed
>
> According to calculations EZNEC had a 40ft top wire. In the field I
> trimmed it until we got an
> acceptable match on several bands
>
> For supports I have used the DK9SQ mast as the main and used the Black
> Widow 20ft
> for the end support with great success
> ----------------------------------------------------
> I use RCA connectors for many of my antennas. I install male RCAs on wires
>
> and interconnect with using DOUBLE FEMALES. For 300 ohm TV wire I will
> solder
> short piece of speaker wire to each of the conductors and then securely
> tape
> the soldered area to the ribbon insulator body . To the end of each
> speaker
> wire I afix a MALE RCA. Generally I will make one speaker wire on each
> end a
> little shorter (3/4 inch) so I can tell easily identify which wire is
> which.
> The RCAs work great and it they break off in the field I carry wire nuts
> for
> emergencies
> ----------------------------------------------------
> I have EZNEC pattern (PDF) for several bands and also a sketch for the
> 300
> ohm connection if you want more details
> ----------------------------------------------------
> Note at the QTH I have used this for 80 and 160m with good success
> including
> contacts with Japan on 160m. I am sure the key is the other fellows has
> HUGE
> arrays but it worked.
>
> Hope this helps
>
> Alan KB7MBI
>
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home:
http://mailman.qth.net/mailman/listinfo/elecraft> Help:
http://mailman.qth.net/mmfaq.htm> Post: mailto:
[hidden email]
>
> This list hosted by:
http://www.qsl.net> Please help support this email list:
http://www.qsl.net/donate.html>
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.html