[CCF] Vertikaali puskissa
ari.korhonen at kolumbus.fi
Tue Oct 14 13:08:03 EDT 2003
160m reflektorilla käytiin ao. keskustelua, joka sai alkunsa siitä kun eräskin kaveri oli pystyttämässä vertikaalia keskelle metsää ja oli huolissaan puiden vaikutuksesta antennin toimivuuteen. Oheisena kaverille tulleita vastauksia.
73 Ari, OH5DX
>Since 1990, I've lived at this location which is 7.5 acres of watershed =
>wooded areas extend for considerable distances in all directions as this =
>town is a
>The woods are rather dense with trees and I had the same questions when =
>I first contemplated
>putting up vertical antennas for 160.
>As has been noted otherwise, I have been unable to find any data that =
>degradation, if any, from trees and foliage.
>Over the years, I've put up several vertical and horizontal antennas for =
>160. This usually
>includes at least two of each type for comparison purposes.
>What I have found, subjectively, is that during the time from the middle =
>of May to the middle
>of November, is that the foliage apparently attenuates the signal about =
>10 dB down
>for my particular situation as compared to non-foliage periods.
>Therefore, during these foliage periods, I always use one of the =
>horizontal antennas and
>don't usually try to chase DX with them.
>My verticals have either been top loaded or, currently, I have a 5/16 =
>inverted L that is about
>90 feet vertical and 70 feet horizontal with 40 elevated radials all =
>mounted in with tree supports.
>On many occasions, I have done comparisons within 500 miles and an =
>occasional DX contact
>during the foliage season and the inverted L is always down about the 10 =
>dB amount previously
>noted as compared to the horizontals in almost any situation.
>I have no way of determining the amount of degradation from the trees =
>themselves even though
>on numerous occasions, because of the amount of ground water, I've noted =
>water oozing out of
>the trees when I install various standoffs to support the radials or the =
>beverages out in the woods.
>This is certainly not the best location for good antenna performance but =
>I have had reasonably
>results during the winter in contests and chasing DX.
>73, Jim, K1PX
>K1PX at msn.com
>The new KC1XX 160m antenna system is installed in dense woods. To get an
>idea of the environment, take a look at some of the photos on the KC1XX Web
>We installed the system over the summer of 2002. If you look at the contest
>results for the 2002-2003 season (CQWW DX, ARRL DX, ARRL 160, and CQ 160)
>you'll see that this system played extremely well relative to the
>competition. We can't give you any quantitive data about how much the trees
>might be hurting, if at all, because there's no way to measure it. However,
>it can't possibly be too much based on the results we've had.
>Bottom line: you deal with whatever the environment is and do the best you
>can under the circumstances.
>73, John W1FV
>Not exactly scientific, but the empirical evidence speaks volumes. Check
>out this guy's "tree antenna."
>It seems to me that if the trees are a significant fraction of 1/4 wL on
>topband, they will absorb power. Whether they 're-radiate' that power,
>well, who knows.
>ford at cmgate.com
>One data point - when W3YOZ ran some (informal) tests a few years ago,
>comparing performance of his vertical in the trees with the one in the open,
>he determined that performance was degraded significantly during the months
>that the trees sap was up (spring through autumn). We theorized at the time
>that trees are like giant columns of water and do absorb MF rf to the tune
>of several db. In the winter when more of the moisture is in the roots, the
>effect is less.
>No, it wasn't a scientific test in the pure sense of the term, but the other
>antenna did provide something of a control and the results were impressively
>consistent from one summer to the next. The trees were very close to the
>vertical, very large (dense), and approaching 100 feet tall.
>(I have observed that my beverage systems (running entirely through hardwood
>forest) seem to be a bit more efficient in the winter too. Just anecdotal
>> Not exactly scientific, but the empirical evidence speaks volumes. Check
>> out this guy's "tree antenna."
>Ford and all,
>The problem is people can say it is a tree radiating, when it isn't. Coax
>with a unshielded center conductor exposed hanging on the tree will excite
>the shield of the coax quite well with even a very small exposed center.
>Antennex has been full of articles about "antennas" that actually use the
>shield as an antenna more than the thing called the "antenna". They hang a
>small "something" on the end, excite the outside of the braid with current,
>and write long articles about how the "thing on the end" radiates!
>In order to be resonant or have length-related resonant effects, the
>resistance of the tree would have to be small compared to reactance per unit
>length. In the early 90's I measured RF resistance of a series of tree
>samples by attaching flat plates firmly against the edges of large cross
>sections of wet fresh-cut trees, and they were many kilo-ohms per foot. This
>included acidic and very sappy Georgia pines, and fast growing trees that
>water would actually pour from (we called them Water Oaks) cut branches.
>Nails would be even worse, because tree cross section contacted by the nail
>would be very small.
>Reports of trees "radiating" lower frequency signals (below upper UHF) are
>largely due to people not understanding how coaxial cable works, and the
>importance of the center conductor hanging out. Trees do reflect RADAR, but
>they are still poor. They are mostly absorbers due to water content, a
>single tree will attenuate a 2GHz signal many dB. The effect that causes
>this rapidly becomes smaller as frequency moves below 2GHz.
>I wonder how people have measured the effects they report, or if it is just
>a feeling based on skywave signal reports. That would be important to know.
> The possible negative absorptive effects of locating an antenna =
>transmitting setup for 160 meters in a dense forest would be very =
>difficult to measure. If the antenna feedpoint impedance changes when =
>the leaves of the surrounding deciduous trees fall off in the Fall =
>season and then again in the Spring season when the leaves grow back, =
>you could presume that some sort of interaction was occurring. But at a =
>wavelength of 160 meters I think losses would be insignificant.=20
> If you set up two identical 160 meter transmitting 1/4 wave vertical =
>antennas one mile apart, one in dense forest and the second one in an =
>open pasture, as a receiving station I think you would be hard pressed =
>to be able to tell the two apart as far as signal strength, taking into =
>account normal QSB.
> As far as actually successfully using trees for transmitting antennas, =
>I think that KF4BWG is being fooled because his BALUN less feedline is =
>doing the radiating.
>Thomas F. Giella, KN4LF
>Plant City, FL, USA=20
>KN4LF 160 Meter Amateur Radio Resources & More:
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