Topband: 160m T-loaded vertical antenna
midnight18 at cox.net
Fri May 30 21:44:54 EDT 2008
Congratulations on your new Antenna!
In your post you said your new vertical has SWR was 1:1 at 1800 and 1.6:1 at
1825 kHz. This would indicate that the top-hat is a bit to long. My personal
suggestion would be to tune the antenna so that self resonance occurs at 1900
kHz. At this point it would only require approximately 2uH of inductance to
move the center frequency to about 1830 kHz.
With a proper ground your 1/8 wave electrical height vertical will present a
very good match to 50 Ohm coax through your 1:4 Unun transformer from 1800 to
1870 kHz. Please note that as you improve your ground system you can expect
the SWR to most likely increase as you reduce the ground losses in that
return path. The best way to deal with that is to use the W2FMI 5 ratio or 9
ratio ultimate match Unun if you are concerned about SWR. The good news at
this point if your SWR does change you are reducing those nasty ground losses
which are probably eating up 30-40% of your transmitted power.
If you should choose to use a little base loading to easily move the resonant
frequency lower use a large diameter (4 in) and ratio of 2-3 turns per inch
low loss inductor of copper or silver plated copper wire of at least 10 gauge
or perhaps 1/4 inch copper tubing, as the feed point is also the point of
maximum current. This can easily be done on a piece of Sch 40 PVC pipe and
can also be your base insulator.
So far you have done a great job of minimizing the losses in the radiator. If
you are using the proper mix ferrite core and have would your Unun per
Jerry's directions you will have a very low loss impedance transformation of
less than .1 db. Do Not compromise your antenna performance by using a tuner.
Why throw away at least 10% of your power in external losses not even
associated with the performance of the antenna or it's ground system?
Again good luck and see you on top-band. Short antennas do work very well
indeed by taming losses.
73 and Best DX
Bob Kile, W7RH
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Apache in charge.
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