I disagree with a couple of conclusions here.
First, if the tower is indeed close to being resonant on 160, then adding
radials (i.e., lowering the ground resistance) isn't going to change the
resonant frequency of the tower (defined to be the frequency where the
reactance of the antenna at the feedpoint is zero). It will REDUCE the
feedpoint RESISTANCE (the sum of the radiation resistance of the tower and the
ground resistance) and will therefore change the SWR. However, note that the
resonant frequency of the tower won't change by adding radials.
Now, if in fact the tower is resonant close to the design frequency of the inv
L, then because of the close proximity of the Inv L and the tower, the tower
will change the feedpoint impedance of the inv L. The easiest way to deal with
this is to de-resonate the tower. Try running a wire from the 60 foot point of
the tower, spaced about 2 feet from the tower, down to ground level. Insert a
variable cap (1000 pF max should do) between the end of the wire and ground.
You can then proceed two ways:
1) Monitor the RF current in the detuning wire while you transmit on the Inv L
with low power at 1830 kHz (or similar). Adjust the variable capacitor to
MAXIMIZE the current in the wire. What you have then formed is a parallel
tuned circuit that effectively decouples the top portion of the tower from the
bottom part and makes it now non-resonant.
2) Just tune the cap for min SWR on the Inv L when you transmit.
Note that options 1 and 2 MAY not yield the same result. That is, in option 2
all you are doing is tuning the entire system so that you have an acceptable
SWR. Wheras in option 1 you are making the tower "invisible" to the Inv L.
Finally, since your inv L is close to the tower, whether you run radials from
the inv L or the tower shouldn't matter too much. The point is to try to get
as many radials laid as you can afford to increase the radiation efficiency of
the antenna.
I had a similar problem here. My 145 foot Rohn 45 is pretty close to being
resonant on 160. My inv L interacted very strongly with the tower. I simply
decoupled the botton part of the tower using the above method and I now have a
very good match.
Note that I cut the inv L on purpose for resonance at 1790 kHz. I can then
tune to resonance anywhere on 160 with a motor driven series vacuum cap located
right at the feedpoint.
Finally, since you have a yagi with insulated elements, I would recommend that
you NOT shunt feed the tower since you will, more than likely, create very high
RF voltages across the insulators between the yagi elements and the boom.
Eventually these MIGHT arc and could eventually be destroyed.
Instead, stick with your inv L idea and de-resonate the tower. (note: in
principle, having your Inv L so close to the tower could also induce high RF
voltages near the yagi, but de-resonating the tower will help reduce the RF
volatage across the instulators).
Hope this helps!
73, Jeff
W3KL
Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402
--- On Thu, 12/4/08, K4SAV <RadioIR@charter.net> wrote:
From: K4SAV <RadioIR@charter.net>
Subject: Re: [TowerTalk] Inverted L Tuning - Solved
To: towertalk@contesting.com
Date: Thursday, December 4, 2008, 11:30 PM
Your last post came in just as I was about to send this one. I see you
found something that lowers your SWR. I will go ahead and send this
anyway, so you can see what was causing the problem.
Assuming you don't have any other wire antennas hanging off your tower,
an 80 ft tower with an XM240 on top should be resonant very close to, or
within, the 160 meter band. This really complicates matters. With only
two ground rods, however, you aren't seeing that resonance. The two
ground rods do not look like a very good RF ground, so the tower
resonance (without the radials attached) is mostly determined by the
tower plus the cables exiting the tower. It not likely that is close to
160 meters.
Now the second issue. Only having two radials on the ground makes them
appear as if they are almost floating. The resonant frequency of the L
will be affected a lot by radial length. The radials are too long to
get a resonance in the 160 band. You really need to add a lot more
radials. That should bring the resonant frequency into the 160 band, as
well as trmendously improve the efficiency. But that's not all the
problem....
Now the complication of attaching the radials to the tower ground. When
you do that, this gives a better RF ground for the tower and now the
tower appears to be resonant in the 160 band. When you try to drive RF
into the L, a lot of the RF is coupled into the tower. The SWR isn't
going to be very low. Short of decoupling the tower, there is no way to
drive the L and not have the tower affecting the SWR. If you move the L
away from the tower, the tower will probably look like a reflector. If
the L is close to the tower, both the L and the tower will act as driven
elements. If you don't connect the tower to the radials, the tower will
not have as much current coupled into it because it doesn't appear
resonant on 160, but whatever current that is coupled into it will go
into the dirt and reduce the antenna efficiency, or be driven down the
cables back to the station. You also really need to connect the coax
for the L to the lightning ground at the tower and not let it float.
Creating a separate lightning ground for the L and then tieing all the
grounds together at the entrance panel might work, but I wouldn't do it.
Did you ever consider shunt feeding the tower?
Note: The balun in the XM240 does not operate very well on 160, which
means the driven element is not electrically floating on 160.
Jerry, K4SAV
RLVZ@aol.com wrote:
>Hi Guys,
>
>Thank You for all the wonderful ideas on getting my 160-m. Inverted L
tuned.
>Your ideas got me going!
>
>My first mistake was connecting the radial ground to my tower ground.
Here's
>what fixed the Inverted L:
>
>1) Disconnected the tower ground from the radial ground and the SWR
improved.
>2) Put in a new 10' ground rod at the base of the Inverted L and tied
it into
>the radial ground and the SWR dropped further to 1.8 @ 1.77 Mhz @ Z=28.
>3) Now that the Z was down where it should be I added the 50/25 ohm UNUN
and
>presto: SWR is 1.06 @ 1.761 Mhz.
>
>Tomorrow I plan to cut the top of the L a little shorter for resonance at
>1.830.
>
>Hope to work you in the ARRL-160 Test this weekend!
>
>73,
>Dick- K9OM
>
>
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