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
|