Dieter commented:
>the problem with the V160 is, that the resistance over perfect ground is
about
>11.5 Ohms. The tuning unit uses - as far as I know - only a loading coil.
>By using this with a good ground system you will never get usable SWR. If
you
>use a bad ground the SWR is OK, but you fire 3 quarters of your power to
heat
>the earth.
>2 Solutions:
>* LC Network for matching (C to ground on the coax-side)
>* 4:1 UNUN between loading coil and coax (option: elevated radials)
Assuming you get the minimum VSWR dipped to the desired frequency (probably
in the 2.5:1 range) AND the impedance of the antenna is less than 50 ohms,
there is another matching option: hairpin coil.
The hairpin will match a less than 50 ohm antenna to 50 ohm coax (step up
transformer). A hairpin coil is similar in function to a straight hairpin
you might see on a Yagi. If your vertical has an impedance of more than 50
ohms, the hairpin will NOT help since it will not step-down impedance.
As discussed, a short vertical should have a low resistance, which will not
match well to 50 ohm coax. When installing a short vertical, the first
thing I always check is the impedance and bandwidth. If a short vertical
has a high impedance, then your ground system is not efficient enough. In
Jamaica we've seen initial impedances of around 80-90 ohms, where it should
have been less than 35 ohms. A 50 ohm impedance on a short vertical might
give you a good VSWR thinking all is OK, but you are loosing transmit dBs
(RX will sound fine). A wide bandwidth on a short vertical is another clue
you have an inefficient ground system.
A recent example was the 160m vertical we installed at 3D2XA. We first
installed a single elevated radial due to space restraints. The 2:1
bandwidth was about 60 kc, which was way too wide for a 53' vertical and
indicated loss. Adding a 2nd elevated radial dropped the bandwidth to
around 25 kc, which was more realistic (calcs show that we gained about 1 dB
with the 2nd radial). We ran out of time and space to install more elevated
radials, but we did put down a small ground screen (not connected to the
vertical/radials). As a comparison, I used the same vertical in Jamaica
last year to set the CQWWCW QRP 160m single band record, and that vertical
installation had a bandwidth of 15 kc! This was the exact same vertical
antenna in both locations. The difference was in Jamaica both radials were
entirely suspended over salt water, and the 3D2 elevated radials were mostly
over (lossy) lava.
Back to the hairpin... The hairpin coil is a step up transformer and is
attached across your hot & ground of your feedline. This is the vertical
section (hot) and radials (ground). The hairpin coil turns should be in the
same direction of the base loading coil, and use solid wire so the coil
keeps its shape. We either use 1/4" aluminum tubing for the hairpin coil,
or copper refrigerator (water line) tubing. 10 or 12 gauge solid wire will
work OK for temporary installations, but may move too easily for a permanent
installation. Starting off with a coil of around 3-4" diameter, and about 6
turns should get you close. Squeeze or spread hairpin coil for best match
(you can easily get a 1:1 match with a hairpin). If your coil is squeezed
all the way for the best (but not yet perfect) match, you will need to add
another turn or two. If spread as wide as practical to get the best (but
not yet perfect) match, then you need to remove a turn or two from the coil.
This is the matching method that Team Vertical uses on all their short
antennas.
Kenny K2KW
Leader, Team Vertical
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|