The new tower will have a log periodic on it with a 32 foot boom (IIRC).
The new tower will have Phillystran guys as does the 100 foot tower.
In Vietnam I use a short MFJ antenna that has a capacity hat that has wires
in a "X" with the a perimeter wire around the "X" much like what you are
describing for some of the AM stations. It actually worked fairly well on
80M for a 20 foot vertical.
Thanks again for the insight.
73, Larry W6NWS
----- Original Message -----
From: "Herb Schoenbohm" <herbs@vitelcom.net>
To: "Larry" <w6nws@arrl.net>
Sent: Thursday, December 15, 2011 7:17 PM
Subject: Re: Topband: Shunt fed tower
> Larry,
>
>
> i think you found the answer that 100 feet with such a large antenna is in
> the BC band somewhere. I was surprised how much top loading I got out of
> my 80 foot tower with a A4S tribander on top alone. A 70 footer is find
> if you have at least a 30 foot boom on top with either a 20 or 15 meter
> beam. If you don't plan to use a beam then you can use the top guys
> connected to the top and put the insulators on each guy at 25 feet, as
> long as the guy angle is not to acute wherein cancellation will occur.
> Some AM stations connect a perimeter wire around the point were the top
> set of guys connect at the insulator. If you do this use power company HV
> insulators. If you use "Johnny Balls for this then use several to avoid
> flashover with high power.
>
>
> The beauty of the cage is that it can be made by smaller wire since it is
> what radiates, not the tower. #6 or #8 power company ground wire (that
> they use at each pole) I have found to be just fine. Put springs of
> turnbuckles, if you want to avid a bunch of standoffs, at the bottom on
> the ground side of the bottom insulators.
>
> If the tower is to long by virtue of having to much top loading, then
> building a decoupling cage is possible but a lot of work. You might
> consider having three 1/4 bottom fed slant wire slopers, individualy
> selected and the tower will give you some directivety as an a periodic
> reflector of sorts. If you do this make sure there is a least 5 to 8 feet
> of distance from the top of the sloper to the actual tower. if not things
> start to intercouple and matching at the bottom is more problematic. Nice
> thing about a bottom fed quarter wave sloper, with some nice radials, is
> that in using three rope halyards you can fine tune the antenna length or
> if it is to short you can add some series inductance to get the match you
> want. some stations report about 3 db of increase or decrease when the
> slopers are switched but the paterrns are broad enough not to produce any
> deep nuls.
>
>
> Herb, KV4FZ
>
>
>
>
> On 12/15/2011 7:45 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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UR RST IS ... ... ..9 QSB QSB - hw? BK
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