I would ditch the GDO and measure R and J.
On 12/15/11 6:45 PM, Larry wrote:
> Herb,
>
> I tried measuring the tower resonance with a GDO as suggested
> by ON4UN's book but my GDO wouldn't go low enough in frequency
> to find resonance. It appears to be below 1.5MHz (100 ft 45G with
> a KT36XA at 100.5 ft, 80M rotatable dipole at 108 ft, and a 2 el 40M
> yagi at 117 feet).
>
> I was thinking about a 6 wire cage 6 inches in diameter probably about
> 2-3 feet out but I will rethink that in light the comments by you, Carl. and
> Guy. It may need to be further out. ON4UN's book suggests an
> omega match in such circumstances but after measuring with the cage in
> place I'll see what it actually needs.
>
> I am hoping to get a better 160 signal out of the shunt fed tower. I
> currently
> have an inverted V that is OK but certainly is not great. I used to have
> a sloper attached to the 100 foot tower that seemed to work better -
> most of the time - but it was finicky to tune.
>
> I am in the process of putting up another tower which by coincidence(?)
> is 128 feet from the 100 footer but the new tower is only 70 feet. At some
> point I will look at phasing them as a 2 el vertical array for 160. The 70
> foot tower may a bit short for that service. I need to do some reading and
> experimenting before I get there though. The line through the towers is
> pointed right at Europe (NE) or Australia (SE).
>
> Thanks all for the comments.
>
> 73, Larry W6NWS
> ----- Original Message -----
> From: "Herb Schoenbohm"<herbs@vitelcom.net>
> To: "Larry"<w6nws@arrl.net>; "TopBand List"<topband@contesting.com>
> Sent: Thursday, December 15, 2011 5:49 PM
> Subject: Re: Topband: Shunt fed tower
>
>
>> Larry,
>>
>> Because of the complexity of modeling without going crazy, although in
>> simple situations it will get you in the ball park, I would highly
>> recommend a 3 or 4 wire cage space at least 24 inches from the area near
>> the rotor plate on a, let's say, 70 to 80 foot tower. The drop wires
>> should be #8 or #6 copper and tied together in a ring supported by
>> porcelain insulators (PVC not recommended in some circles) around at the
>> base with one wire connected from the ring going to your proposed ATU.
>> With a MFJ bridge measure the feed wire's reactance and impedance against
>> ground. With one climb have your tower climb buddy work his way from the
>> top in 2 foot increments jumpering the cage to the tower with large
>> alligator clips (nothing fancy for this purpose) and tell him or her to
>> keep coming down until you get close to 50 ohms. (It can be 40 to 60 ohms
>> as that is sweet point enough you me) Then back a better connection using
>> split copper bolts with three jumpers to the tower. Whatever the
>> reactance is you can tune out that inductive reactance with an equal value
>> of capacitance. As Guy said forget about the tower being resonant
>> anywhere since in such circumstances you may never get that. A tap coil
>> to ground will get you with a simple L network and series cap should get
>> your SWR to 1:1 even if the sweet point is a bit off. Again the components
>> should be, flat wound coil with correct tap connections, a vac of at least
>> 750pf with broadcast mica paders if required for more C.
>>
>> I have found that single wire shunt feeds are the most problematic to work
>> with, especially when the beams are on multiple levels. A larger diameter
>> cable, if you must only use a single wire shunt can be obtained from using
>> a length of CATV .750 which is 3.5 inch in diameter. But a big shunt
>> doesn't look all that hot and a three wire cage is beautiful, looks like
>> it will work, and in fact does.
>>
>> Good luck,
>>
>>
>> Herb Schoenbohm, KV4FZ
>>
>>
>>
>>
>>
>>
>>
>>
>> On 12/15/2011 5:14 PM, Larry wrote:
>>> I haven't done much modeling in the past.I have a KT36XA which would be
>>> very
>>> ugly if I had to model it precisely. I also have a linearly loaded 2 el
>>> 40M
>>> yagi.
>>> I suspect that the loading wires probably are negligible in the overall
>>> scheme
>>> of things at 160M. So I would guess that there some approximation that
>>> would
>>> give reasonable results as a place to start on the tower. Suggestions?
>>>
>>> 73, Larry W6NWS
>>> ----- Original Message -----
>>> From: "W2XJ"<w2xj@nyc.rr.com>
>>> To:<topband@contesting.com>
>>> Sent: Thursday, December 15, 2011 10:34 AM
>>> Subject: Re: Topband: Shunt fed tower
>>>
>>>
>>> Vertical antennas have been shunt fed for over 70 years. There is no
>>> magic involved. Very few MW verticals are ever resonant and resonance is
>>> irrelevant. The only important thing is to match the TX so it is happy.
>>> The easiest way to deal with matching is to first model on EZNEC which
>>> will give an approximation of where the shunt should be connected and
>>> then physically moving the shunt to find the 50 ohm point which should
>>> be determined by measurement. Once that is accomplished, measure the J
>>> and calculate the necessary C to cancel it.
>>>
>>> On 12/15/11 10:17 AM, Joe Subich, W4TV wrote:
>>>> On 12/15/2011 7:27 AM, W2RU - Bud Hippisley wrote:
>>>>
>>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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