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VK2TV > 6M 19.10.08 23:10l 64 Lines 2812 Bytes #999 (0) @ WW
BID : 47021_VK2TV
Read: GUEST
Subj: Re:6Meters Ver Antenna
Path: DB0FHN<DB0MRW<DK0WUE<7M3TJZ<F6CDD<F6BVP<VK2TV
Sent: 081019/2058Z @:VK2TV.#MNC.NSW.AUS.OC #:46684 [Kempsey, QF68JX] $:47021_VK
From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
To : 6M@WW
# Generated by: TstHWin v2.21b - Registered to VK2TV
# On : 10/20/2008 7:48:48 A
# UTC: 10/19/2008 20:48:48
>From: VK2DOT@VK2DOT.CC.NSW.AUS.OC
>To : 6M@WW
>
>Hi All, many years ago, one used to use a mobile 6 meters coaxial dipole.
>Howevere the cost of Al & other items has made it not worth wile anymore.
>So wonder if anybody has the plans for a good vertical 6meter antenna
>constructed in a poly pipe aprox 9-12 feet long. Not a J pole, orplus no
>external ground plain radials. Possiably made out odf coax in the poly pipe.
>Where the antenna can be used mobile or portable in operation.
>Regards Rod vk2dot$tpg.com.au ......////
Hi Rod & all,
You are obviously looking for some kind of Ground Plane Independant (GPI)
antenna, which means you are looking for something that is voltage fed,
i.e. an end-fed half wavelength radiator.
The Zepp feed (think J-pole) is one way to achieve a match to such an
antenna but the additional quarter wavelength make for an unwieldy mobile
structure, it being some 15+ feet in length.
Another way of feeding such an antenna is with an LC network (which the
Zepp feed is anyway). Think halfwave 27MHz CB antennas or Ringo for
practical examples. This requires a parallel resonant tuned circuit that
inserts between the lower end of the radiating element and ground. The
coaxial feed is tapped up from the grounded end to achieve a match. The
capacitor is usually, but not necessarily, a coaxial device formed by the
lower end of the radiating element inserting into an insulated bushing in
the lower metal mounting tube. Insulator quality is important due to the
high impedence at this point.
Commercial versions of the GPI use a series coil to ground from the bottom
of the radiating element. The capacitive component is provided by a series
capacitor between the coaxial feed cable inner conductor and a tap on the
coil. My home made 2m version of many years ago used a 4.7pF capacitor.
A couple of Australian manufacturers made GPI antennas for the 70-85MHz
Land Mobile band. Both used a loading coil to reduce the physical length of
the antenna. One design used a loaded, fibreglass whip approximately 5 feet
in length which mounted to a tubular PVC mount with metal end caps. This
housed the matching network. The other design used stainless steel whips
with a centrally (approximately) mounted loading coil between the top and
bottom whip sections. The matching network was mounted in a fibreglass tube
that was fitted to a PL259 connector to provide the connection to the base.
Type two permitted the entire antenna to be removed, type one did not.
I used both of these antennas on work vehilces, the need for a GPI arising
when we bought some trucks with fibreglass cabins.
Cheers ... Ray vk2tv
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