Something I read somewhere after trying to get the resonance is that with the GDO you might not get a good reading if there are BC stations close by. I don't think there are any BC stations that close to me but I did not investigate.Given the discussion here I had figured to read R+jX. I didn't think to do that before. The resonance with a GDO I think is actually attributed to W1FB (SK). It is just a single wire about 2 feet out from the top of the tower to the bottom with a small loop for the GDO at the bottom.
I tried the GDO about 2 years ago BTW. I have been working on the yagis which got torn up by a tornado this past Spring. Once I get the yagis fixed and back up I can get on to the shunt fed activities. My son normally climbs for me since the Dr grounded me but my son went "lame" a few weeks ago. All of this may have to wait for Spring. 73, Larry W6NWS ----- Original Message ----- From: "W2XJ" <[email protected]> To: <[email protected]> Sent: Thursday, December 15, 2011 6:59 PM Subject: Re: Topband: Shunt fed tower >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 > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
