A half-wave doublet center-fed with a quarter-wave of hi-Z feedline is a worst case scenario.
A 66' doublet would be close to a resonant antenna on 7.0 MHz. Accounting for the thin wire, would expect a 70 ohm center-feed impedance, even at that height. It would appear that the length of the feedline would be approaching 30 feet if not over. Without calculating this exactly, given the thin wire and 3" spacing, the feedline characteristic Z would be over 600 ohms, perhaps as much as 800. The feedline will be very close to a 1/4 wave transmission-line transformer, given some velocity factor adjustment. The worst case of this scenario is to raise the feed impedance at the K3 end of the feedline to 800 squared divided by 70, or 9000 ohms, an enormous mismatch to the design 200 ohms on the antenna side of the balun. At 5 watts, the antenna tuner/balun combination would have to generate 212 volts on the feedline to match 9000 ohms. Even with moderation due to not being exactly at the worst case points, you could have had a 10:1 or worse mismatch at the OUTPUT of the balun, taking it way out of design parameters and efficient operation. In this case the rf path would have been taking any old alternative way to induce current, including capacitance of the rig to the ground below and dissipation in pure heat. The reason your balun was hot was that a lot of the transmitting power was being dissipated in the core. For forty meters using the open feedline you would try 66' of wire and the feedline off-center at 22' from either end, or the feedline at the end of a 43' wire. Your open wire feed will ENABLE these lengths. I assume you have the auto-tuner in the K3 or you would not have been able to get power out. Further elaboration off-line. 73, Guy K2AV Dave VW wrote: > > I apologize for the bandwidth, but I really trust the sources of info here > and it may help someone else starting out like me with specific K3 info > and > antenna theory. > > I've been playing around with my K3 in a portable capacity out in my yard. > Here is the setup: > > -K3 running @ 5W > -Antenna: 66' doublet made from 28g wire fed with open feedline made from > same, spaced apart with wood dowels 3" wide. It is placed up in a tree > about > 35' up with the ends coming down in an inverted v fashion. > -The antenna is fed to a BL2 with a 12" piece of coax to the K3. > > I used a similar setup last summer with my K2 and had what I thought was > pretty good success. In trying to evaluate this setup, I usually call CQ > as > opposed to answering one. This way I know that if I get a reply, the other > op heard me. During the exchange I will state my power info to which I > usually get positive comments about the K3 and/or the antenna setup - or > they quickly sign off (not sure why they do that if I got a 599 report). > > Yesterday, I was tuning around 40m when I heard a station calling CQ - QRP > on 7.059. He was a 479 here so I thought I would give him a call back. He > answered and gave me a report of 359. After the initial exchange, he > stated > he could not copy me - we tried a couple more times and then signed off. > He > was a definite solid copy on my end with the S-meter hitting S1-3 at times > with some QSB. It was like this the whole time. > > Now my questions: > 1 - with my antenna setup tuned with the KAT3, knowing that we were both > at > 5 watts, is it possible for the antenna to be more efficient at receiving > than transmitting? I guess I always thought that if an antenna "system" is > matched then the energy transfer is reciprocated for rcv and xmt. > > 2 - with all the great receiver reports I've read on the K3 - could it be > that I was simply hearing him better with my K3 than whatever he was using > (unfortunately I don't know what he used as we never got that far in the > QSO)? > > 3 - I did notice after the QSO that the balun was a little warm. If there > are transmission losses in the balun, would there not be reciprocated > losses > on the receive side? > > If you feel this is not list appropriate, please reply to me privately. > > Thanks for the bandwidth. > > 73, > Dave W8FGU > > > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > > -- View this message in context: http://www.nabble.com/OT%3A-K3-and-antenna-questions-tp18240875p18242928.html Sent from the Elecraft mailing list archive at Nabble.com. _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
