"EZNEC analysis of the antennas shows no difference with the reflector
grounded or not."
I should have added some details to that statement, so that I don't
mislead anyone. It's true that the XM240 shows no difference in its SWR
or radiated pattern with the reflector grounded or not, but there is a
difference in possible antenna interaction. If you mount either a 20
meter or 10 meter antenna, close to, and at right angles to the XM240
such that there is a large coupling of signal into the boom of the
XM240, there will probably be significant interaction. This is because
with the reflector grounded to the boom, there is a resonance that
develops close to 20 meters and one within the 10 meter band. There is
no problem if the antennas are mounted with their booms parallel to each
other. There may be a problem if you have another antenna close and it
can be independently rotated.
As it comes from the factory, with the reflector isolated, and another
antenna mounted at 90 degrees, a similar problem should exist on 15
meters. With the reflector isolated, the driven element should be
isolated by the common mode choke (balun), and that leaves just the boom
and its parasitic capacitance across the isolators. This should
resonate close to 15 meters.
I have not measured any of this data. It all came from simulations.
Jerry, K4SAV
K4SAV wrote:
>"I have never understood why the 40-2CD reflector is grounded and the XM240
>is not........basically they are the same antenna...."
>
>I'm not really sure either, but I'm willing to take a guess.
>
>When they did the mechanical upgrade and added the long U channels to
>support the elements, of course they had to isolate the driven element,
>and if the reflector was also isolated, then both elements can be
>identical except for the extra length required of the reflector. If they
>had mounted the reflector without the isolators, then it would be
>connected to then entire length of the U channel, this would lower the
>resonant frequency of the reflector because there would now be a section
>with much larger diameter (the U channel). It's a lot easier to
>manufacture identical elements than to make the two different. Of course
>they could solve this problem by keeping the isolators for the reflector
>and then adding a single strap to the boom at the center, but this is
>more hardware, and I'll bet they couldn't see any reason to do it.
>
>The reason I added this strap was because I didn't like the reports from
>others of arcs jumping across the reflector isolators during close
>lightning. Since this gap is only about 1/8 inch around the U bolts,
>the arcs would be depositing metal fragments across the insulators at
>these points. When this kind of thing happens over a period of time. I
>have found that a short usually develops. I don't like shorts or
>intermittents showing up at various places on my antennas. If there was
>just a gap without the insulator, then there would be no place for the
>arc to deposite the metal dust, but this is not the case.
>
>EZNEC analysis of the antennas shows no difference with the reflector
>grounded or not.
>
>Jerry, K4SAV
>
>
>Robert Pack (NX5M) wrote:
>
>
>
>>IF the reflector is grounded to the boom would the actual length of the
>>reflector have to be changed a little?
>>I have never understood why the 40-2CD reflector is grounded and the XM240
>>is not........basically they are the same antenna.
>>
>>Still scratching my head on this one.
>>
>>Bob NX5M
>>
>>----- Original Message -----
>>From: <k2qmf@juno.com>
>>To: <nx5m@txcyber.com>
>>Cc: <towertalk@contesting.com>
>>Sent: Wednesday, April 12, 2006 8:30 PM
>>Subject: Re: [TowerTalk] New Cushcraft XM240 Trouble
>>
>>
>>
>>
>>
>>
>>>Try grounding the center of the reflector as is
>>>done in the 40-2CD...
>>>
>>>73, Ted K2QMF
>>>
>>>On Wed, 12 Apr 2006 19:42:02 -0500 "Robert Pack (NX5M)"
>>><nx5m@txcyber.com> writes:
>>>
>>>
>>>
>>>
>>>>I am out of ideas so let me probe the minds here. I have already
>>>>been in touch with CC on this issue.
>>>>
>>>>Bought a new XM240 a couple of months ago. Built it to specs for
>>>>the center of the band. The antenna did not present an swr any
>>>>lower than 1.6:1.
>>>>This past weekend the antenna was taken down and up several times.
>>>>The coils and balun were checked but nothing seemed to be abnormal.
>>>>All connections are good. Measurements were triple-checked before
>>>>the antenna went up two months ago but we checked them again just to
>>>>make sure. All are correct. We even changed the tip sections to
>>>>the upper part of the band setting but still saw the same
>>>>thing.......best swr 1.6 (only higher in the band as it should be).
>>>>The coax was tested and the antenna was even tested with a different
>>>>piece of coax. No luck at all. Finally the balun was bypassed and
>>>>the coax connected directly to the driven element but that did not
>>>>seem to make much difference.
>>>>What I do find interesting and confusing is that at the lowest swr
>>>>point, or resonate spot, the analyzer is telling me that it is 29-30
>>>>ohms. This I do not understand.....but it does explain the
>>>>mismatch. I also do not understand why the analyzer shows 50 ohms
>>>>at another point in the band but the swr is over a 3:1.
>>>>Now CC wants me to send the coils back for testing which I guess I
>>>>will do since the antenna is on the ground anyway and somewhat
>>>>useless to put back into the air given the swr issue. All of this
>>>>even after CC sent me two new coils just in case I found a bad
>>>>one........which to the best of my knowledge none of them are bad.
>>>>There are very few failure points on these antennas so I am really
>>>>confused and frustrated.
>>>>Based on the 29-30 ohm reading I guess I could always feed the
>>>>antenna with two 130 foot runs of 75 ohm cable in
>>>>parallel........just being silly. But spending what I had to spend
>>>>to buy the antenna seems the thing should be better than this. The
>>>>40-2CD that is on the top of the tower works just fine (flat swr at
>>>>the design frequency).
>>>>To me it has to be a coil or the balun.
>>>>
>>>>Anyone have any helpful ideas?
>>>>
>>>>Thanks.
>>>>
>>>>Bob NX5M
>>>>
>>>>_______________________________________________
>>>>
>>>>
>>>>
>>>>_______________________________________________
>>>>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
>>
>>
>>
>>
>>
>
>_______________________________________________
>
>
>
>_______________________________________________
>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
|