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"<[email protected]> > To: "Larry"<[email protected]>; "TopBand List"<[email protected]> > 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"<[email protected]> >>> To:<[email protected]> >>> 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 > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
