Short version: Anyone who has four elevated radials can easily check
this out. How to in Long Version.
1/4 wave elevated radials can have 18-60 ohms ground induction loss
per radial. Divide by number of radials for series loss.
1/8 wave elevated radials can have 5-12 ohms mostly ground induction
loss per radial. Divide by number of radials for series loss.
A single elevated 5/16 wave folded counterpoise will have 2-3 ohms
mixed wire and ground induction loss. +/- 32 feet.
Long version:
Nobody is telling you to take down your radials or modify it if you're
happy with it. Far from it. If you like it then of course you keep
it. Go work 'em and have a great time. God bless you and have a
great life and all that! But people are asking for advice here...
1) What comes up over and over and over and over in correspondence
(way, way more times than you guys that are happy with yours) is that
people don't have the space for traditional radials and what they have
tried just plain sucks (their appraisal, not mine). In my case I do
have the acreage for dense elevated radials other than the highly
inconvenient fact that a drive way goes right through the middle of
the available space and either elevated and buried radials are out of
the question. What then? What FACTS to design an alternative to all
of the traditional radial answers? What would be the tradeoffs? How
to measure? Is there anything possible besides 5 buried 15 foot
radials I already know suck big time, that I can try? Exactly what is
it that makes that makes 160m antennas so unbelievably bad?
2) What does someone DO who is trying honestly to compare alternatives
and the FIRST TIME AROUND squeeze the last dB out of what is going to
be a considerable project, because they don't have the time to try
five different things, one at a time, each one occupying all available
space? Even if they did it, what beyond QSO count affected by highly
variable conditions would they have to compare sky-wave effectiveness?
To your considerable credit, 227 countries on 160, even using an
amplifier, probably proves the worth of the operator more than the
antenna. I've always said that there's anywhere between zero and 27
dB gain possible between the ears. If you've worked 227 countries I'd
suspect you're one of those 27 dB gain between the ears guys. That's
the inverse of the old saw "What do you call a lid with an amp and a
great antenna? LOUD lid." "What do you call a top op with 100 watts
and a piece of wire? DXCC"
Doesn't it bother you just a little bit that there's nothing out there
carefully verified that would allow you to calculate these kinds of
160m sky-wave things? There hasn't been any serious money into
research of these issues in our lifetimes. The commercial MF
interests are 99% concerned with ground wave which doesn't do us any
good. The buried radial aspects of MF vertical performance were well
settled in the 1930's by Bell Laboratories and legendary figures of
the time.
FCC regs put the old research in concrete. A station builder buys the
property, constructs the antenna with dense buried radials per FCC
rules, marks it all up as a capital investment in their tax records
and moves on to worrying about programming and making a profit. They
are not funding research projects to see if there is a way to improve
sky wave, or see how to do improve sky wave with elevated radials, or
see how to accomplish efficient sky-wave with something other than
radials.
Since you already have four radials up and ready, here's a
straightforward experiment you can do with them. Temporarily separate
the radials from each other and any connection to
ground/shield/matching network connection. Now just four 131' wires.
Call them North, South, East and West for illustration purposes,
whether they are or not, just so N and S are opposite wires in line
and E and W are opposite wires in line.
Connect the N and S wires together. Connect the E and W wires
together. Connect the N/S junction to the center conductor of a PL259,
and the E/W junction to the PL259 shield using the SHORTEST
connections possible, as in couple inches, preferably less.
Put an analyst on that directly, or with no more than an inch or two
of coax/coax connectors (very, very important). Support the analyzer
in some non-conductive manner away from misc conductors or capacitive
coupling to the analyzer case, or anything metal and then orient it so
you can read it without touching it. Read the resistive component of
the impedance. What is your R at that point? If you look at the
series/parallel nature of how the radials are fed, you will see that
the feed R you are measuring is also the average individual radial R
over your dirt.
Try feeding the N/S pair as a dipole. Dividing this by two will give
you a worse number, even though 5 to 10 percent of that power is
actually radiated. So multiply half the dipole R by 90% and it's still
probably a worse number.
A NEC4 model of of the cross-fed 1/4 wave radials in FREE SPACE (just
the radials) shows a feed R of about 5 ohms, radiation from this
configuration at 1.98 dBi peak, and an average gain of -1.91 dB. This
loss would be wire loss and high circulating currents as that's all
there is left to make loss. The 2:1 tuned bandwidth SWR span of only
8 kHz shows how very high Q it is.
On the other hand, model the same exact wires 8 feet above one of the
Norton-Sommerfield grounds in NEC4, and it's behavior completely
changes.
Over one particular ground simulation, the radials' feed R is now 21
instead of 5, peak radiation is now -17.2 dBi instead of +1.98,
average gain is now -22.79 dB (yes, MINUS twenty two point seventy
nine), and the 2:1 matched SWR is now roughly 60 kHz instead of 8 kHz.
This last is the classic broadening of a high Q circuit's bandwidth
by ADDING LOSS.
Models of other earth characteristics will fetch up to more than 60
ohms feed R per radial. Just depends on the dirt.
Given all the above, how can you have the change if there is no
current induced in the dirt?. Why do we NOT put dipoles "up" at eight
feet over ground? Just exactly WHY does a dipole at eight feet
totally suck? It sucks for the same reason 4 radials in a cross-fed
configuration at 8 feet somehow come up with 21 or more ohms of series
resistance that is almost entirely loss in the ground. 8' high
cross-fed radials, 100 watts, -22.79 dBi, is an ERP of one half watt.
1500 watts to the wires results in an ERP of 7.5 watts, or QRP
radiated. This is REALLY leaky dummy load grade absorption. There is
current in a resistor somewhere and all it can be made of is dirt.
I can't tell you why you never heard about ground induction (induced
ground currents) in school. The polite answer would be by some other
term? The cynical answer would be that they didn't know that current
was induced in dirt, and so they couldn't put it in the book. Except
Hal Beverage (and those other legendaries back when this WAS a hot
subject for commercial money) documented current in the ground.
You can do the experiment above. Then you have a quantifiable
characteristic of an elevated radial that shows 20 plus ohms ground
induction loss per radial, where it is only mitigated by adding more
identical radials in parallel.
If one is so unlucky with THEIR dirt to hit one of those 60 ohm jobs,
then their four elevated radials in parallel will have a 15 ohm loss
to go in series with their 15 ohm radiation resistance antenna, and
measuring a total of 30 ohms and a nice wide comfortable SWR curve,
think they are in 160 meter heaven, never knowing that of that 1500
watts, that 750 watts has gone to ground induction loss in the
radials, and of the 750 left, 325 went below the horizon and was lost
to reflection loss we can't do anything about, leaving only 325 watts
spread around the horizon.
If you were lucky enough to have a 20 ohm per radial figure, you COULD
do as little 5 ohms, which would put 562 watts around the horizon.
The comparable four times 1/8 wave elevated radials, shows less than 5
ohms R per radial, for a series loss of 1 or 2 ohms. That means you
put 700 watts around the horizon. You have to tune the radial left
over reactance out with an inductor, but why isn't everybody saying to
ditch 4 full length elevated radials for 1/8 wave. (Secret: SWR is
broader with 1/4 wave radials. Good SWR equals good performance,
right? Except WHY is the SWR broader....)
And for those poor souls who don't have the 90 foot square on their
property to even do 1/8 radials?
There is a single elevated 5/16 wave folded counterpoise that comes in
at 2-3 ohms series loss and only needs + and - 32 feet from the feed,
and has no more or no less reflection loss than either four 1/4 or 1/8
wave radials. You only need one 5/16 folded counterpoise. That will
be up on a web page soon.
If you can do the experiment with your 4 1/4 wave radials, the results
would be interesting.
73, Guy.
On Mon, Oct 25, 2010 at 11:04 PM, Hardy Landskov <n7rt@cox.net> wrote:
> Guy,
> All I know is I worked everything I heard with 4 elevated radials. I even
> went to 8 elevated radials and no difference on transmit.
> What is induction loss? Never heard that in all the college antenna courses
> I took. And yes 99% of 160 antennas are not optimum by any stretch of the
> imagination. But we do what we can and try and get some fun out of it. So
> don't grouse about it.
> I have 227 countries on that band from AZ so something is working.
> Hardy N7RT
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