When one uses a very short vertical radiator, rarely it happens that a good
and extensive RF ground system is instead available.
All considerations about nearly good grounds and antenna loading are more
academy than anything else if such a very good ground is not really
available in practice.
In case of poor or fair ground, typical of ham installations with
restrictions, any attempt to reduce ground losses and to rise the antenna Rr
is surely raccomendable and do changes overall efficiency in terms of
several dBs.
Under this perspective, I wouldn't care much of antenna Q unless it leads to
exaggerated narrow BW or corona effects.
73,
Mauri I4JMY
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <TOWERTALK@contesting.com>; <n4kg@juno.com>
Sent: Sunday, July 07, 2002 1:24 PM
Subject: Re: [Towertalk] Re. HF2V
> > N4KG: For the most part, this is true, until you reach 250-280
> > and have to work over the poles to finish off the rest, THEN
>
> I've rarely is even worked over the poles on any band below 40
> meters. For example, the normal path from here to Mongolia is SE in
> the evening and SW in the morning. Same for deep Russia and India on
> 160 and 80 meters.
>
> > transmitting performance makes the difference between getting
> > there and NOT. When I added my 130 ft high dipoles, Central
> > and SE Asians became MUCH easier to put in the log.
>
> Everything is all relative to what we compare it to.
>
> > > One reason so many people have dismal results with base-loaded
> > > antennas is they swallow the "BS" about only needing a few radials.
> > >
> > N4KG: True. KZ4V had 120 radials under her base loaded HF2V
> > with NO top loading wires. The radials were probably in the range of
> > 35 to 40 ft which admittedly reduced performance on 80M.
>
> There is a problem tossing around anecdotal evidence to determine
> performance, and we should all be aware of the problem.
>
> Factually, once .025 to .05 wl spacing is reached adding more wires
> does no good at all. So 50-60 radials would have worked just as well
> on 40 meters, and on 80 she might as well have had 15-30 radials as
> 120.
>
> It would not sound impressive if we said "she had 15 1/8th wl
> radials", yet the system would be about the same as 120 radials the
> same length.
>
> The person listening to the story hears "120 radials" and hears
> "empty lot", and doesn't think about the fact the system was actually
> a modest or poor radial system and there could have been any number
> of problems with the rest of the installation, such as site clutter.
>
> It is only on 20 meters and higher than the 120 radials helped, and
> only then a fraction of a dB over having 50 or 60 radials.
>
> > The REAL convincing observation was that she moved from being
> > one of the LAST to get through the pileups with her HF2V to being
> > competitive in the pileups after her 90 ft tower was installed with
> > the inverted vee and vertically polarized Delta Loop. These antennas
> > were obviously MUCH more competitive in the pileups.
>
> That isn't necessarily a problem with the HF2V, and we should all
> know better than to leap to that conclusion based on the data.
>
> Had she A-B'ed a full size vertical in an open location at the same
> site under the same band conditions and seen a change, it would
> indicate an efficiency problem with the vertical system, location,
> or ground system. Changing to a dipole (especially when comparing
> anecdotal data to anecdotal data taken at a different time) proves
> nothing, because it could be any number of things at work.
>
> One thing often overlooked is the distance required to minimize
> pattern distortion. On 160 meters, a tower 500 feet away from my omni
> vertical changes the pattern about 10dB in four directions! On 80
> meters, think of all the vertical or near vertical metallic
> objects...let alone all the other variables at work.
>
> Now I know the Gap vertical is no good, because I tried one with a FS
> meter and had a good ground system under the reference antenna. i
> also A-Bed them.
>
> Factually, an MFJ 80/40 top loaded vertical and a Butternut were
> about even...and the Gap was 10-15 dB behind. How many countries I
> worked on each or how they worked in pileups is immaterial, because
> of all the inconsistencies in other factors important to results.
>
> > > Factually, there is almost no difference between base and top
> > > loading if the ground system is good. Top loading does provide
> > > about
> > four
> > > times the radiation resistance of base loading, but top loading
> > > generally has (for a given radiator size) less bandwidth.
> >
> > N4KG: HUH? Since when does higher radiation resistance result
> > in "LESS BANDWIDTH" (assuming the same height of radiator) ?
>
> In heavily loaded systems with fixed external losses, bandwidth is
> mainly set by reactance limits. The antenna can be viewed as a series-
> resonant L/C system. Anything that decreases the amount of reactance
> increases bandwidth, because it decreases system Q.
>
> For example, my narrowest mobile antennas are the antennas with the
> SMALLEST amount of capacitance above the loading coils, regardless of
> efficiency or radiation resistance.
>
> As a matter of fact the widest bandwidth antenna I have has the
> highest efficiency. You have to look at the entire system, and NOT
> just relate bandwidth to one factor.
>
> The MFJ vertical is VERY narrow in bandwidth because it has a small
> top-hat, and large inductor. The same antenna has nearly eight times
> the bandwidth when base loaded, and if the ground system is near-
> perfect efficiency can actually INCREASE.
>
> The top loading makes the system more forgiving of poor ground
> systems and allows you to use the antenna on 40 meters, but it also
> makes the system have LESS bandwidth.
>
> > > If the ground system is poor, top loading can reduce loss by up to a
> > > factor of three or four. If the ground system is good, efficiency
> > > changes are often not measurable.
> >
> > N4KG: What value of ground resistance is "good"?
> > What does it take to reach this level?
>
> Ground loss would have to have a normalized value (at the feedpoint)
> of about 10% or less of the other resistances in the
> system...including radiation resistance...in this example. So 40 1/4
> wl radials would work in most cases.
>
> Brown Lewis and Epstein did all this in the RCA report "Ground
> Systems as a Factor in Efficiency". Their conclusion, and one that I
> have verified, is that with a good ground system you might just as
> well base load the antenna.
>
> > > Most of the DXCC totals do with hours operated, location, and
> > > operating skill. I'd say transmitting antenna performance is way
> > > down the ladder, unless someone really has a poor antenna
> > installation.
> >
> > N4KG: Again, basically true UNTIL you start talking about polar path
> > contacts.
>
> Polar path contacts virtually never occur on 80 or 160 meters, and
> rarely occur on 40.
>
> 73, Tom W8JI
> W8JI@contesting.com
>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and take an
additional 5 percent off
> any weather station price.
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
|