The TH7 was designed to cover ALL of the 10, 15, and 20M
bands. Unfortunately, this resulted in a SEVERE GAIN SLOPE
on 10M, rising 3.2 dB from 28.0 to 29.7, making it a Killer on
10M FM but worse than a 2L Yagi for CW and SSB !
I found the following info for changing the DE / SWR
on 10M but have not been able to find the changes
to the REFLECTOR and DIRECTORS needed to improve
the gain on 10M in the CW / SSB portions of the band.
I know the information exists, but I must have misfiled it :-(
--------- Forwarded message ----------
Date: Wed, 29 Apr 1998 11:50:29 -0500
Subject: [TowerTalk] TH7DX Gains
Steve's recent post which included the gains of the TH7DX made me
I found the original project book on the TH7, dated May 21, 1981. My
measurements on the TH7, dated Sep 4, 1981 show the following gains:
14.000 MHz 5.0 dBd
14.175 MHz 5.0 dBd
14.350 MHz 5.2 dBd
21.000 MHz 5.8 dBd
21.200 MHz 6.1 dBd
21.450 MHz 5.7 dBd
28.000 MHz 3.8 dBd
28.800 MHz 5.5 dBd
29.700 MHz 7.0 dBd
These measurements were taken on our rotating pole as seen on the cover
QST Feb 1985. A transmitter at the south end of our property (aprox 2500
feet distance) generated the signal. The receiver was the same as used
radiation patterns. The meter was calibrated against a calibrated step
attenuator. The TH7 was replaced with a resonant dipole for each of the
three bands, and a monitor antenna was also used to ensure a stable
during and between the tests. The measurement uncertainty in these
measurements is likely on the order of 0.5 - 1.0 dB, but the numbers
above are my original measurements on the first prototype. Better
(on the order of 0.5 dB) could be obtained by using a directive gain
reference such as an NBS yagi, and by constructing an ideal HF range free
As was pointed out earlier, an amplifier can produce more "gain" than any
single antenna. However, the antenna also helps with the received
both in gain and discrimination (F/B, F/S and takeoff angle).
73 Roger WB0DGF
Telex Communications, Inc.
Fred Hopengarten K1VR
Six Willarch Road * Lincoln, MA 01773-5105
781/259-0088 *eFax 419/858-2421
You may find this old note from Roger Cox useful. He's no longer with
HyGain, and HyGain is now in MS.
From: "Roger A. Cox WB0DGF" <firstname.lastname@example.org>
Date: Mon, 24 Mar 1997 12:04:36 -0500
Subject: [TowerTalk] TH7DX once more
The TH7DX first sold in 1982. The dark aluminum is not a problem on the
exposed surfaces, it is likely oxidation. When you take the antenna
clean the mating surfaces (inside and outside) with a Scotchpad,
steel-wool, or other type of light abrasive until the metal is shiny.
Penetrox-A or another type of anti-oxident (for aluminum) on these
when re-installing. ARRL has a list of available anti-oxidents.
The hardware used on the TH7DX has always been Stainless-steel except for
the 4 long boom-to-mast bolts. There was a conversion kit (product 392s)
available from 1982-1986 which allowed a TH6DXX to be converted to a
This kit also included all new SS hardware. Sometime in the mid-80's,
SS U-style element compression clamps were replaced with standard all-SS
compression (hose) clamps. Unless the SS bolts have galled or stripped
threads, these do not normally have to be replaced. Use heavy motor oil
SS threads to prevent galling. The 4 long boom-to-mast bolts were
originally cadmium-plated, but are currently SS.
Inspect the trap-caps. If this is an old antenna, or if it has been in a
very sunny environment these black plastic caps on the trap ends will
replaced. Also, the outboard caps should have a 0.25 inch drain hole in
the sloping surface for water drainaige. This hole needs to be lined up
with the drain holes in the trap body, and directed towards the ground
installed. Inspect the screw inside one end of each trap. This
self-tapping screw shorts the outer capacitor shell to the inner tubing
the inboard end of the trap. Before abt 1990, this screw was plated. If
SS, then tighten. If plated, you may wish to replace. If you see
of bugs or insects inside the traps, use compressed air to blow out their
nests, etc. If you have a grid-dip meter, check the traps for resonance.
The meter will show a resonance of approx. 27.6 MHz on the short 10 meter
traps and 20.6 MHz on the 15 meter traps.
The monoband elements of the TH7DX used a plastic rope in the tips to
prevent or reduce wind vibrations. This is recommended.
The TH7DX comes with the BN-86 balun. This may need to be replaced if
damaged. We also recommend upgrading to the BN-4000 or Palomar
or homemade RF choke if very-high power is to be used. Also it is
recommended for club stations or multi-op stations where lots of
people operate. (The BN-86 may arc if used on the wrong band with high
power). The BN-86 is rated at 1500 watts PEP with VSWR less than 2:1.
In the 1990's we added a "DX" setting to the TH7DX. RDE-2=33 inches (838
mm) and FDE-2=42 inches (1067 mm) this lowers the VSWR curves on all 3
bands but does not affect the gain or patterns. New resonant freq's are
14.140, 21.250, and 28.950 MHz. The bottom edge of each band ends up at
approx. 1.5:1 VSWR.
Roger WB0DGF (Hy-Gain)
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