I just had an interesting antenna experience.
K4JRB mentioned that his XM240 works as well as his dipole L (I think
maybe that is an inverted L?). Anyway, knowing that an antenna this
short can't be very good on 160 meters I ran it thru EZNEC and came up
with a feedpoint impedance of 2-j2100. Running this number thru TWL I
get a loss for my 225 feet of RG213 of 34 dB. Seems reasonable. So this
antenna should be lousy.
Tonight I tried it. I used a tuner to match the XM240 to 50 ohms on 160
meters. That was the first surprise, the tuner was able to match it.
Then I compared it to a low 160 meter inverted vee (apex at 50 ft, ends
at 15 ft). At first I compared it to close stations that would likely
arrive at high angles. To my amazement, there was no difference, both
had about the same signal strength. Then I listened to some European
stations. Another surprise, the XM240 was as good as the inverted vee,
actually better because the noise level was much lower on the XM240.
I know there must be a logical reason for this, and with antennas
analysis programs, it is usually because you are not analyzing the real
world situation. This was the case here also. Actually, the real
antenna is not the XM240. The XM240 has a balun, however at 160 meters
it probably doesn't work as a balun at all. With the feedline taped to
the boom, mast and tower, plus the shield is connected to the tower,
this antennas is very unbalanced, and the common mode currents go right
into the tower, and other lines. So the real antenna is the tower. In
my case, the 160 meter inverted vee is also on the tower, and this also
radiates some. So with the real model now in hand, running the impedance
again, I get a feedpoint impedance of 800 -j1150. Doing a plot I get
about the same gain as the low inverted vee. Plugging the impedance into
TLW, I get a feedline loss of 5.8 dB. So the calculated difference now
is within 5.8 dB. Refining the model further would probably resolve the
remaining difference, (it's probably the other antennas on the tower
which I didn't include in the model) but the real question has been
resolved. The XM240 does not work well on 160 meters, but the feedline
for it does.
Jerry, K4SAV
David Thompson wrote:
>I used the beam as a RX antenna along with either a shielded link loop or
>K9AY thru a Palomar pre-map with ability to switch between two antennas. I
>have used it for transmit in the early 1990's ams agian several years ago
>this time in low power thru a runer uisng my club call W4BCV. I remember
>the first time I used it in the mid 80's. I was putting up a 160 loop but
>the ARRL DX SSB was on and HC8DX was 59+ on 160 SSB. I tuned up my amp thru
>a tuner and he said I was the loudest USA there. I also worked several in
>Europe on the beam over the years. My raw SWR is about 4 to 1. I find that
>with my antenna switch in line that 100 watts is the max before the switch
>or beam starts shifting SWR.
>
>It is not as good as say the K9AY for RX as it picks up static as well or
>better than say an Inverted L. I have a short Dipole L and the L is quieter
>but local sigs ate louder on the beam. In the ARRL DX SSB I listened to
>PI4TUE and G4BUO. Both were S9 to 10 over on the L and S7 to 8 on the beam.
>K9HMB was 30 over on the beam and only 15 over on the L.
>
>My results agree that the straight up gain is quite good
>and not as good at 40 degrees which Western EU usually comes in at. Not
>sure about the 34 DB loss in RG 213 coax. I use 9913 to my switch and 12
>feet of 213 freom the switch to the beam. It sure beats the old 80 meter
>dipole with shorted coax trick tho.
>
>73 Dave K4JRB
>----- Original Message -----
>From: "K4SAV" <RadioIR@charter.net>
>To: "David Thompson" <thompson@mindspring.com>
>Sent: Thursday, March 09, 2006 12:26 AM
>Subject: Re: [TowerTalk] XM240 INTERACTION WITH TRIBANDER
>
>
>
>
>>I just looked at my XM240 in EZNEC at 1.8 MHz and the gain, straight up
>>with the source at the antenna, didn't look too bad, but when I figured
>>in the line loss for my RG213, the line loss was 34 dB. Now maybe if I
>>could open the shield connection.....
>>
>>It does work as a 160 receive antenna (better than a low dipole) but not
>>nearly as good as an EWE, K9AY, or beverage.
>>
>>Jerry, K4SAV
>>
>>David Thompson wrote:
>>
>>
>>
>>>My old 402CD is set for mid band. SWR is 1.5 at the top and 1.7 at the
>>>bottom.
>>>
>>>I helped as friend set up his XM240 to the same setting and he finds
>>>
>>>
>almost
>
>
>>>the same.
>>>
>>>Even at 35 feet (Tower cranked down for rotor repair) its a killer...ask
>>>N4KG or K4BAI.
>>>
>>>Plus it works well on 17 meters. K1MM uses his on 12 but I find its too
>>>much of a cloverleaf (great or bust) and unpredictable. A secret that
>>>
>>>
>few
>
>
>>>know...it works as good on domestic work as most other antennas on 160
>>>
>>>
>but
>
>
>>>thru a tuner. I have a dipole L
>>>and you cannot tell the difference on stations within 1200 miles. Now on
>>>
>>>
>DX
>
>
>>>it falters but hey an antenna you can use on 4 bands...I forgeot to say
>>>
>>>
>that
>
>
>>>I switch between my KT 34XA at 76 feet and the 40 at 86 feet and often
>>>signals on 15 are louder on the 40. Again a tuner is required. So a 40
>>>meter beam that works like gangbusters on 17 and 15 and is adequate on
>>>
>>>
>160
>
>
>>>for domestic work and can work 12 in a pinch. Now to see if will work on
>>>80...
>>>
>>>Oops did I let a secret out??
>>>
>>>73 Dave K4JRB
>>>
>>>
>>>_______________________________________________
>>>
>>>
>>>
>>>_______________________________________________
>>>TowerTalk mailing list
>>>TowerTalk@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>>>
>>>
>
>
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|