Night Owl 6/Night Owl's Shareware - PDSI-006 - Night Owl Corp (1990).iso/035a/discones.zip/DISCONES.TXT

Text File | 1991-01-12 | 6KB | 108 lines

Article 7292 (236 more) in USENET>rec.ham-radio:
From: [e-mail] (Ron Parise)
Subject: Discone Antennas
Message-ID: <[e-mail]>
Date: 24 Oct 88 13:27:08 GMT
Sender: [e-mail]
Organization: The Internet

Discone antennas are indeed very impressive for just about any
frequency range. I do not believe that one can be build which will
cover 25-1300Mhz as RS and others claim. A 10 to 1 bandwidth ratio
is all that can be expected from the design. The low end cutoff
frequency is determined by the length of the cone (slant length).
The cone is 1/4 wavelength at the lowest operating frequency. The
response of the structure degrades very rapidly below that freq.
The length of the elements on this recent flood of discones on the
market is such that it's low end cutoff should be about 100MHz. You
might also notice that they specify that it can be used for transmitting
on 144,220, and 440Mhz, even though they claim it is good down to 25.
They do not claim it can transmit on 50Mhz, which supports my claim
that it just won't work below 100. I think they just claim the wide
bandwidth to make it appeal to the scanner crowd.

I started playing with dicones about 10 years ago and I currently have
on that covers 48-480MHz, just right for 50, 144, 220, 440, and
everything in between with < 2:1 SWR. I would like to build an HF
discone for 7-30MHz at some point. They give excellent low radiation
angle response over the whole HF spectrum and are really not that big
(relatively speaking).

Building discones is quite easy as they are not particularly critical
in their construction requirements. You can find a set of simple
cookbook type formulae in the radio amateurs handbook for designing
your own.

Happy disconing!
Ron




From: [e-mail]
Subject: Discone antennas (and Ethernet coax)
Message-ID: <[e-mail]>
Date: 25 Oct 88 12:24:16 GMT
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Organization: The Internet
Lines: 58

        There was an article in 73 Magazine a year or so ago (I'll see if I can
find the exact issue) which described how to build a discone for 30 - 1200 Mhz.
Since the discone only has a working frequency range of about a decade, the
author designed it to cover 120 - 1200 Mhz and added a cut-down helical CB whip
to the top of the antenna. This whip, together with the mounting pole, formed a
coaxial dipole for 30 - 50 Mhz.

        I built it. It's made of 1/8" brass welding rod (about $6 worth), has 8
radials (in both disk and "skirt") and the hardest part of building it was
getting the place where the radials connect machined. It would appear that it's
not possible to build a discone without at least a good drill press & maybe a
lathe. Anyway, I got a friend who works in the machine shop at MIT to do it for
me. The article had all the details & he just whipped up the pieces for me out
of scraps around the shop.

        It works fine. I have a VHF - UHF TV mast mounted preamp at the antenna
(covers from channel 2 to old channel 83, about 56 - 900 Mhz or so) to fight
feedline loss. I have a friend who used one of these on his Grove Scanner Beam.
They have a lousy noise figure (about 6 db is typical) but plenty of gain so
you can use cheap coax.

        The RSGB VHF - UHF handbook has drawings for another version of the
VHF - UHF discone (I like their junction hardware a little better but it's
tougher to machine). I've looked at commercial discones (I spotted one  on a
building at Spaceport USA when I was in Florida this spring and counted
radials as the tour bus drove by - I believe there were 8 but I've seen them
with 16). It was made of welded aluminum, painted white. About 1/2" - 3/4" dia.
radials.

        Also in Florida, just outside Ft. Lauderdale, there's a commercial HF
radio facility in a field next to the highway (talk about antenna farms!). I
made my wife take a detour and did some quick photography. Lots of rhombics but
the most impressive thing I saw was an HF discone. Picture a 60 ft dia ring of
60 ft tall phone poles with a wire connecting their tops in a circle. From this
circular wire, about every foot or two along its circumference, wires  run down
to a plate about 6" off the ground in the exact center of the circle of poles.
(there's your cone...I assume the disk was buried under the soil). There's a
horrendously expensive book ($80), put out by Artech House (Norwood, MA) called
_Shipboard Antennas_, whic is basically a catalog of Navy antennas with all
their specs. It's chock full of discones (I looked at it at a recent trade show
& would have bought it if it wasn't so overpriced as to be silly!).

        Anyway, good luck; they're not that hard to build & they do seem to
work pretty well. No way is it worth the $70 that Heath and Radio Shack are
charging & it is possible to build it yourself. Besides, you get to have people
stopping by your house and asking why you put your Christmas tree on the
roof...

        BTW, Ethernet coax: The teflon-coated kind seems to be
pretty resistant to weather. PL-259 connectors fit perfectly & the loss figures
are similar to RG8 (1.0 dB/100 ft at 50 Mhz, compared to 1.2 for RG8). The
latest Belden  Master Catalog (885) has the loss characteristics on page 186
(up to 50 Mhz.)

73 de KA1AXY
Pete Simpson
[e-mail]