N4KG response inserted below.
On Tue, 10 Sep 2002 Jon Ogden <na9d@speakeasy.net> writes:
> on 9/10/02 10:08 AM, n4kg@juno.com at n4kg@juno.com wrote:
>
> > You do NOT want any conductors over 12 ft long between
> > insulators anywhere near an antenna for 10M. Note that
> > the first insulators must be less than 5 ft from the tower
> > to meet this requirement since there is continuity through
> > the tower. (N4KG)
>
> That's not quite correct. If you look in the ARRL handbook, you can
> find a lengths of cable that will be fine. In fact, the ARRL says that
25
> feet or so is about the ideal length.
That information was first published at least 30 years ago,
LONG before computer modeling was available, and has been
MISINTERPRETED for decades.
First, there is NO SUCH THING as a NON-RESONANT conductor length.
A 25 ft conductor is self resonant near 20 MHz and makes an
excellent REFLECTOR on 15 Meters. A 27 ft conductor is
self resonant around 18 MHz (The 17 Meter Band) and can
act as a sub-optimum Reflector on 10, 12, and 15 Meters
and a sub-optimum Director on 20 Meters.
What is NOT generally understood is that ANY Conductor
that is over 3/8 wavelengths long at the frequency of interest
WILL couple to another (Driven) Element. I learned this when
modeling a 2L 6M antenna.
A parasitic element spaced at 0.15 WL acted as a DIRECTOR
when tuned between 0.3 and <0.5 WL.
It acted as a REFLECTOR when tuned from >0.5 WL on out to
MANY Wavelengths long, truncating with 2 dB gain and 4 dB F/B.
I have posted this information several times on TowerTalk.
If you don't believe it, model it for yourself. I think you will
change your opinion about "non-resonant" guy lengths !
FWIW, I have steel guys and insulators on all 7 of my towers.
I placed the First Insulator 3 to 4 ft from the tower to minimize
the conductor length immediately under the antennas taking
into account that there is continuity THROUGH the tower
from insulator to insulator.
I follow this with (typically two) 12 ft lengths which are nearly
invisible below 30 MHz. These are then followed with combinations
of 27 ft and 40 ft lengths which are spaced at least 27 ft from the
tower, but I now suspect may still have some interaction on the HF
bands. N4ZR has reported that 20+ ft of insulated guy between
from is 40M Yagi was NOT sufficient to prevent interaction with
(continuous?) steel guys extending from that point to ground.
This is why so many of the BIG GUNS are going to Philystran
or Fiberglass guy rod.
Tom N4KG
As you get longer and longer, it gets
> harder
> and harder to get a length of wire that is not resonant on
> frequency.
>
> >
> > I would use Philystran or some other NON-STRETCH
> > non-conductor such as a rope that meets these requirements.
> >
>
> I have used regular steel aircraft wire (purchased from Menards or
> some
> place like that) on my roof tower for the last 5 years. It has
> worked great
> and is still in excellent shape.
>
> The anchor points in the house roof are green treated 2x4's about
> 2.5 feet
> long that have been lagged into the roof. I've withstood winds of
> at least
> 90 mph with that setup and no problems whatsoever. N roof tower is
> a 14.5
> foot tripod and until recently had by TA-33, a UHF beam and a 2m
> Ringo on
> it. The rotor was a Ham IV. I am in the process of erecting some
> Rohn 25
> for that stuff and the roof tower will hold my satellite antennas
> from now
> on.
>
> > How much guy length could you possibly need on a roof top?
> > How cheap do you need it to be?
>
> I had a chat with the sales guy from Phillystran at Datyon this
> year. It
> was an enlightening conversation. Kevlar guy wires are not any
> stronger
> than steel. I had thought you could always use a smaller diameter
> Kevlar
> cable than steel - not true. For the Kevlar cables, you will need
> to use
> "big grips" and not cable clamps.
>
> The main thing the sales guy told me was that if I was planning an
> installation that I would have up for a long, long time - say more
> than 10
> years, then I should seriously consider investing in his product.
> But he
> said that if I am not necessarily planning on having my setup for
> that long,
> it wouldn't be worth it. The advantages of Kevlar is that it
> doesn't rust
> and is non-conductive. It is about 4 times more expensive.
>
> For a roof tower application, there is no reason why you shouldn't
> be able
> to find a wire length that is not resonant at HF and guy your tower
> with
> that length.
>
> 73,
>
> Jon
> NA9D
>
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Life Member: ARRL, NRA
> Member: AMSAT, DXCC
>
> http://www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
>
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