Interspersed.
On Wed, Dec 12, 2012 at 7:10 PM, DAVID CUTHBERT <telegrapher9@gmail.com>wrote:
> Guy,
>
> here is where I believe your mysterious extra "loss" in NEC is coming
> from. You are reading the "average gain" loss. NEC calculates that by
> integrating the power at infinity and dividing by the power into the
> antenna. This accounts for the far-far field ground losses that vertically
> polarized radiation encounters.
>
Yes, indeed.
But where do those ground losses ACTUALLY start? Why is it that what you
call "far far" ground losses can vary by raising very low feed points small
amounts above ground that could not possibly be pattern changing in the
distance? What you call "far far" losses start at the BASE of the antenna
IF the lossy dielectric of the earth is NOT effectively shielded by an
opposite and equal counter-field supplied by dense radials (why proper
radials are best). This is a kind of loss that Roy Lewallen terms
underestimated in NEC4.
If one understands that the losses to ground in the distance start at the
end of a dense radial field, and beyond that point manage to lose 3.9 dB
for a 1/4 wave system, then what happens to the level of loss when the
radius or density of the shielding effect is reduced. What if the
shielding extent and density is reduced to NOTHING by use of a ground rod
as a counterpoise?
When all of this was being shaken out, nobody with money was carefully
calculating the Sommerfield ground estimation method for a BC tower
connected to a ground rod, or ten foot radials. Gotta remember all that
stuff was NOT written for hams, NOT written for us and our faint imitations
of proper commercial solutions. NEC is just not calibrated for a lot of
the stuff that we try over really dreadful dirt. We're just hoping that
the extrapolation down into too-small attempts holds. The extrapolation
seems to have some severe problems.
>
> But, this is not how we report the radiation efficiency of a vertical.
> That is defined as the radiated energy in the far field (but not too far)
> divided by the power into the antenna.
>
Doesn't a term such as radiated energy in the near far field have to run in
lock step via formula with the EZNEC loss figure? Using mV/m figures at a
mile or a kilometer is FAR, isn't it?
And if it doesn't track, is the term worth anything for real application?
The range of the integration is the same for both processes, and with one
degree points in EZNEC the difference should be way down in the decimal
points.
And are you saying that the 3.9 dB out there isn't actually lost, that it's
really there somehow because its not seen in something called radiation
efficiency? The experience says that the loss is there, and the bulk of
anecdotal material says the loss can be worse than calculated.
I'm just saying that we mark as risky, possibly really bad, anything not
reasonably close to full size, dense and uniform all around, or not
specifically designed to operate without commercial radials with specific
attention to minimization of ground induction from counterpoise and
vertical radiators.
Faint imitations of radials are going to cost us loss, and the really
unfortunate ones can send us down toward 20 dB. The sooner we figure out
exactly why the better. Denial doesn't cut it anymore. RBN is double
blind. RBN is essentially verifying the anecdotes rather than
contradicting them.
73, Guy.
> Dave WX7G
>
>
> On Wed, Dec 12, 2012 at 4:34 PM, Guy Olinger K2AV
> <olinger@bellsouth.net>wrote:
>
>> [I may have sent an incomplete version of something on this topic.
>> Apologies]
>>
>> 6 dB would be someone's calculation based on currents. The sometimes
>> abysmal performance of a ground rod based vertical system cannot be
>> explained by 6 dB. Cutting my amp from 1500w to 375watts just doesn't get
>> bad enough. Not close.
>>
>> Part of the problem is that we figure all the current in the vertical is
>> free and clear to useful radiation and is not affected by the RF
>> "appearance" of the ground.
>>
>> You can model a near perfect commercial grade radial field, with a radial
>> system apparent series resistance of a few tenths of an ohm, and NEC4 will
>> still come back with an overall loss of 3 to 4 dB. There is no
>> book-keeping of that loss in the 34 ohm vertical radiator in series with
>> the near zero resistance of the radials, as people typically talk about it.
>>
>>
>> If I run my EZNEC Pro NEC4 3D all points azimuth and elevation run for a
>> gold standard commercial installation, 120 1/4 wave buried radials in
>> "average" ground, at the bottom of the main window I will get a rather
>> typical 3.9 dB overall loss. IF we have to understand loss as only
>> book-kept in the feed resistance numbers, one would have to APPORTION the
>> 34 ohms resistance to account for the loss. That would be 47 percent in
>> the pattern and 53 percent lost somewhere, by some means, 16 ohms
>> apportioned out to the pattern with 18 ohms of loss somewhere, but not in
>> the radials and not in the vertical wire.
>>
>> Great radials. Top of the line radials. BUT there is still some
>> mechanism draining off 53 percent of the power. The math in NEC 4 is
>> doing and sensing something that explains loss not book-kept in our
>> all-loss-is-shown-in-the-feed Z mental picture.
>>
>> Where's the loss, loss that does not change the feed Z as it comes and
>> goes. How does it work? Can this non-Z-changing loss increase without the
>> commercial radials and a wire fed right at the ground? One could picture
>> my installing a really good antenna at a place looking at a fairly distant
>> horizon. However, I could have trucks and bulldozers built a 1000 foot
>> high dirt wall encircling my place a mile away, and my feed Z would not
>> change. The wall would just soak up RF that otherwise would be out doing
>> wonderful things at low angles.
>>
>> How much additional does "unshielded" dirt underneath a naked vertical
>> soak up in terms of dB that does not alter the feed Z?
>>
>> Persons unnamed in 1995 recommended two 1/4 wave radials on the ground.
>> I remember that K4CIA from the other side of town, running QRP on his well
>> constructed 160 vertical could often beat me out running 100 watts. What I
>> had was like running QRP on a good antenna.
>>
>> We don't know everything. And there are a lot of people that have awful
>> results with hack job radials. We need to quit recommending hack jobs
>> until we know exactly why they do or do not work, and can explain a FAR
>> greater percentage of the anecdotal material than we can now, and can
>> explain how in some scenarios how we can get results that ARE plainly down
>> 20. If we're not careful, at some point we can be blowing off the
>> essential majority story because we just don't want to listen.
>>
>> 73, Guy
>>
>> On Wed, Dec 12, 2012 at 3:17 PM, DAVID CUTHBERT
>> <telegrapher9@gmail.com>wrote:
>>
>>> Correction, 100X the loss.
>>>
>>> The deal difference between a single ground rod and a BC station ground
>>> will be about 6 dB.
>>>
>>> Dave WX7G
>>>
>>>
_______________________________________________
Topband reflector - topband@contesting.com
|