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Re: Topband: 160m T-loaded vertical antenna

Subject: Re: Topband: 160m T-loaded vertical antenna
From: RK <>
Date: Sat, 31 May 2008 01:44:54 +0000
List-post: <">>
Greetings Jo,
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
Linux is like living in a teepee. No Windows, No Gates,
Apache in charge.

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