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Isotron 15m (modified) antenna

Is this a magicmiracle, mystery, or just a miserable antenna that radiates from the feedline?  Who knows.  Is it a usable and portable antenna?  Well ... it worked for me with some careful preparation and setup.

Isotron 15 meter antenna design enhancements

An Isotron 15 meter antenna with counterpoise copper wires replaced by thick grounding cable and aluminum plate:



Cable & label:



Aluminum counterpoise conductor:



Field deployment at SLAC:



 ARRL International DX phone contest contact:

15m Isotron voice contact

ARRL international DX phone contest on March 6, 2010:
unconfirmed contacts with UA0FM (Russia), LT1F (Argentina), JA8RWU (Japan) using 80 watts.

Equipment: Isotron 15m antenna, 12 ft. painter's pole, 1:1 current choke, LDG YT-100 tuner, and Yaesu FT-857D.

My very first PSK31 contact

I made my very first PSK31 contact, with AA9GR at 17:50 UTC on Jan. 22, 2010 using 20 watts. The Isotron 15 antenna is mounted indoors on PVC pipe.

Equipment:
  FT-857D / LDG YT-100 tuner / fldigi on Slackware 13 Linux.

The antenna can also be suspended by running a thin rope through the antenna pipe, and then anchoring the ends of the rope between two tress like a dipole.


Deployment of the Isotron 15m antenna at Agnews Historic Park, Santa Clara, CA on March 28, 2010 for the CQ World-Wide WPX Contest 

Unconfirmed contact log (replying to CQs):

Local time
(24 hr)
UTC Callsign His
RST
My reply: His
RST & seq #
Repeat
my call?
Frequency
& mode
Transmit
power
Location via
QRZ.com
Comments
 15:48 PDT
 22:48 Z
 VK4KW  59  59 #001
 no  21.319.0 USB-PH
 60 W
 Australia  7100 miles
 15:58 PDT
 22:58 Z
 NX5M  59  59 #002
 no  21.344.5 USB-PH
 60 W
 Texas  1500 miles
 16:03 PDT
 23:03 Z
 KZ2V  59  59 #003
 no  21.379.2 USB-PH  60 W
 Stanford, CA
 12 miles - K6TT confirmed (by email) signing as KZ2V @ W6YX
 16:13 PDT
 23:13 Z
 ZX5J  57  59 #004
 several  21.263.0 USB-PH
 95 W
 Brazil  6500 miles - signal had some fading
 16:18 PDT
 23:18 Z
 LW1DTZ  59  59 #005
 once  21.245.0 USB-PH
 80 W
 Argentina  6400 miles

Contacts shown on Google Maps using KML  <-- broken no longer working method

HF RIg: Yaesu FT-857D in a Pelican case, a modified Yaesu MH-59A8J microphone, and a LDG YT-100 auto-tuner.

Note: only the LDG YT-100 auto-tuner was used for tuning - the tuning elements on the Isotron antenna itself was untouched when making contacts on different frequencies.  Therefore, it is unknown what part of the band the antenna was actually tuned for - but based on the log, the antenna might be tuned more for the 21.3xx range than for the 21.2xx Mhz range.
For the next outing, will try the following:
    • confirm the "Isotron radiating feedline theory" by placing a snap-on ferrite choke at various places along feedline and checking the SWR
    • tune the antenna by adjusting the counterpoise arm and coil tuning plate (when the LDG YT-100 tuner is in bypass mode)
    • use a 15 meter Dunestar 300-21 bandpass filter
A fellow ham suggested that this particular (15m) antenna design with the coil & counterpoise arm is essentially a tuned LC (tank) circuit, and probably has a radiation pattern which would be more isotropic in nature, feedline excluded.

All the equipment is laid out on the picnic table:


The operating outpost (with an airplane taking off from the San Jose airport):

[ The transceiver is grounded through an Alpha Delta TT3G50 surge protector, thick ground cable, and tent stake ]


Close-up of the Isotron 15m antenna on a 6-to-12 foot Mr.LongArm Alumiglass 6512 extension "painter's" pole:

[ Yellow nylon guy ropes stabilize the pole to 3 ground stakes.  A 12 foot feedline connects the antenna to a 1:1 current air choke sitting at base of the painter's pole. ]

DO NOT OPERATE IN ANY POTENTIAL LIGHTNING WEATHER CONDITIONS!


The 7 foot ground cable, with battery clamp at one end, is loosely clamped around the feedline:

[ The other end of the ground cable is fixed to the Isotron counterpoise arm ]


Army tent tripod:

[ The white PVC pipe coupler provides a snug fit to a painter's pole - and ground stake holes are drilled in the tripod paddle feet ]


Google Maps satellite view of the operating outpost:



Isotron 15m antenna SWR measurements - taken at Stanford Linear Accelerator Center (SLAC) next to the Panofsky Auditorium


April 24, 2010:

Setup and measurements, using MFJ-259B (thanks to KN6QI):



May 15, 2010:

Setup for best SWR, ground cable wrapped around aluminum section of pole, and keeping the feedline away from the ground cable:


Very good SWR achieved, but is bandwidth narrow? - using FunkAmateur FA-VK MKII vector analyzer (thanks to K9WS):