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Re: Topband: 160m T-loaded vertical antenna

Subject: Re: Topband: 160m T-loaded vertical antenna
From: <>
Date: Wed, 28 May 2008 10:13:57 -0700
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Hello Jo,

> Last weekend, I finished installing the T-loaded vertical antena using the 
> 18m Spiderbeam fiberglass pole. It has 3 wire spokes for top load, each at 
> 15 meter long. 

Congratulations on your new antenna.  N6BT, KE7X and I installed a very similar 
antenna for the 2007 CQWWCW contest in C6A (C6ARR on 160m).  It should be a 
very good antenna for you.

Our installation was a 55' tall vertical with 4 top-hat loading wires (also 55' 
long), which came down at rather steep angles due to the limited area we had to 
work with.  We also installed 2 elevated radials, and then laid down about 20 
ground screen radials of various lengths.  the vertical was mostly 2" thin-wall 
tubing, tapering to about 1" tubing at the top.

Without any loading coils, it resonated somewhere in the 1.920 MHz range, which 
is VERY good for such a short antenna.  A relatively small amount of base 
loading was added to bring the antenna to resonance.  The base loading coil was 
made with 1/4" aluminum tubing.  We use a small base loading coil on 
DXpeditions to make tuning in the field a bit easier.  It would be better if 
you can resonate your antenna using the top hat wires only.

>The SWR 1,6:1 at 1825 kHz. The lowest 
> 1:1 is at 1800 kHz. I think I must pull out radials in straight-and-level 
> form to obtain the SWR match and resonance (at 1825 kHz) and will see if the 
> radial wires would need trimming.

For an electrically short antenna like this, this SWR curve seems too wide.  I 
would expect a 2:1 SWR width on the order of +/- 25 kc.  Your 50 kc 1.6:1 SWR 
bandwidth would indicate some losses in your system.  I would suspect your 
radial system (first), or your main vertical radiator width.  

To check your radial system, look at the feedpoint impedance:  a short vertical 
like this should be in the 10 ohm range (check without your W2FMI matching 
transformer).  If the impedance is higher, like in the 30-60 ohm range, that is 
a key indication of too much ground loss.

If so, one way to reduce the ground loss and increase efficiency is to add a 
bunch of ground radials and do not connect them to the elevated radials or the 
vertical.   I would start with maybe 12 ground radials, and then re-check your 
SWR curve.  If the SWR curve gets narrower AND the feedpoint impedance drops 
(both of which are signs of a more efficient short vertical), that will again 
tell you there is too much ground loss with the 3 elevated radials.  Hopefully 
you can leave the ground radials in place, and add more if possible.  I guess 
you could try adding another elevated radial, but I would first suspect your 
existing radials have too much coupling to ground.

If you are using a wire for the main vertical radiator, that too will reduce 
efficiency.  Adding 2-3 more vertical wires around the sides of the pole and 
connecting them at the top and bottom of your pole will effectively increase 
the diameter of the vertical radiator.

Another efficiency point for elevated radials is the width of the wire.  I 
would use at least #12 gauge if possible.  Using a small diameter wire will 
reduce efficiency, and limit how much power you can run.  Modeling also 
suggests that using really large diameter wire (or tubing) for the first 40-50' 
(if I recall) of each elevated radial will also increase efficiency.  This 
could be a good use for that old coax you have laying around.  

For both the main vertical, and the elevated radials, the wider the radiator 
for the main current part of the antenna, the better the efficiency will be.

> This is my first time to build vertical antenna for TX on topband. 

You are off to a great start!  I would first look at the feed point impedance - 
I think you have too much ground loss.

Best of luck,  Kenny K2KW

P.S. the antenna we installed in C6A crushed the QRP world record on 160m in 
the CQWWCW contest.  We did have the advantage of having the antenna on a small 
bluff about 20m from the ocean with water path to northern JA, USA, EU, and 
northern AF).  But the best QRP QSO C6ARR/N6BT had was over land to VK6HD.  
This was QRP!  Mike sure has great ears.
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