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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