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Author Topic: Hygain 18ht hytower 160 meter kit high vswr  (Read 2672 times)

Posts: 16

« on: December 16, 2012, 05:51:52 PM »

I put my 160 kit on my hytower 18ht this weekend. It is the highpower trap version that mounts on the top of the tower and is basically an inverted ELL. The tower is close on the other HF bands to where it was before. With some minor shifts. However the 160 portion is returning a high vswr. It appears to give a vswr dip at 1900 to about 4 to 1 vswr. I have 24 radials under the tower ranging from 130 feet (4) to about 35 feet. Random lengths. Feeding it with Heliax cable. I have usually had good sucess with Ell antennas and find this one a little puzzling. I plan on doing an impedance sweep to see what I really have. I am wondering If I am having a trap issue. Any thoughts.
Paul, K8IO

Posts: 17483

« Reply #1 on: December 16, 2012, 08:09:59 PM »

If the SWR dips at 1.9 MHz it is unlikely that there is anything wrong with the trap or
the antenna tuning.

If the 160m adaptor attaches to the top of the tower portion (rather than the top of
the 80m tubular element) that puts it something like 35' off the ground.  An inverted
L with a 35' vertical portion on 160m will have a feedpoint impedance around 7 ohms.
Add 5 ohms of ground loss and you get 12 ohms, for a 4 : 1 SWR.

It sounds like the antenna is working exactly as intended.  If you had a poorer ground
the SWR would be better, but of course the efficiency would be lower.

If you attached the trap and loading wire to the very top of the antenna the input
impedance over perfect ground increases to about 16 ohms.  With the same 5 ohms of
ground loss that gives you 21 ohms, for an SWR of about 2.4 : 1.  But that might not
be practical from a mechanical perspective.

Posts: 17483

« Reply #2 on: December 17, 2012, 08:49:11 AM »

Try adding a 2.4uH coil across the feedpoint at the base of the antenna
and see if that helps.

Posts: 960

« Reply #3 on: December 18, 2012, 07:30:12 AM »

Try adding a 2.4uH coil across the feedpoint at the base of the antenna
and see if that helps.
That's a game saver at times. Like a small transformer.
If you have an MFJ 269 it will show what component of the SWR is making your readings high. Capacitive reactance or inductive reactance. The MFJ 259 can be helpful too but there's more a of a trick to it.

Posts: 17483

« Reply #4 on: December 18, 2012, 10:02:34 AM »

And for those of us who were experimenting with antennas before SWR analyzers
or antenna modeling software appeared on the market, there is a lot that you can
figure out with just simple measurements.

In this case, with the dip to minimum SWR at 1.9 MHz, we know that is very close
to the resonant frequency, so the reactance component is very small.  With an SWR
of 4 : 1, that means the resistive component is either 12.5 ohms or 200 ohms.
Unless the ground system is extremely poor,  the circumstances would point to the
former value.  The ARRL Antenna Book (13th edition, 1974) has a convenient chart
on page 122 showing the shunt reactance to match various impedances, and it looks
like 20 ohms should be about right.  This could be either a capacitor or an inductor
with appropriate retuning of the main element, but an inductor will cause less
disruption on the other bands.

So all we needed was the minimum SWR value to make a pretty good estimate of
the required matching coil value.  Of course, the SWR reading may not be precise,
but that should give us a good starting point for further adjustment.
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