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"starman" wrote in message ... Dave wrote: replies interspersed "starman" wrote in message Since you're using twinlead as the horizontal antenna element, there is no point in keeping the two wires in the twinlead seperated at either end. The twinlead will act like it's one wire if you connect both of it's wires together at each end. The near end (both wires in the twinlead) will go to one wire of the high impedance side of the matching transformer. Um, couldn't I get more signal out of a longer wire? And wouldn't I have that if I connected the conductors on the far end, and left one hanging out in space on the near end? The two wires in twinlead are too close together to act as a loop or a longer antenna wire when you connnect just the far end wires together. If you want a longer antenna you would have to use a longer length of twinlead. I forgot to mention in another post that the high side of a shortwave balun should be about 500-ohms. This is another reason why the homemade transformer is better. It's wound for an impedance ratio of about 9:1. The TV balun in 300-ohms on the high side. Okay, but I don't understand why I would want a 500 ohm impedance matching transformer on a 300 ohm line. Would the impedance mismatch not result in an attenuated signal, or signal loss? You don't have a 300-ohm line. The charecteristic impedance of a single wire antenna like the inverted-L or random wire is in the range of 400-600 ohms depending on it's height above ground. You are trying to match that impedance range to the 75-ohm coax. For some reason, the fog is clearing today. I now remember reading this somewhere else, but forgot it. I follow you now. I figured you were going to use just a horizontal antenna section with the coax going up to one end. That end is where the matching transformer should be. Gotcha. That's where I was going to put it. The TV balun should have two wires on one side (300-ohms) that may look like a short length of twinlead or they could just be two pigtails. One of those wires goes to the end of the antenna, which in your case is the two wires of the twinlead connected together at that end. The other wire of the 300-ohm side must go to the coax shield. That's why some kind of adapter is needed with a grounding screw on the 75-0hm side which has the threads, so you can connect the remaining 300-ohm wire from the other side to the coax shield and threads via the screw. The 300/75 ohm matching transformers I have at this point have pigtails on the 300 ohm side, and a shield and center conductor on the 70 ohm side (a female F connector.) If I am not mistaken, one of the pigtails correlates to the center conductor of the female F connector, and the other correlates to the shield. In other words, I think it's already set up that way, I don't actually have to connect to to the shield. Am I wrong here? That's not the way a balun is made. The two 300-ohm pigtails go to the high impedance winding on the ferrite core in the balun. This winding is isolated from the low impedance winding that goes to the 75-ohm female F-connector. That's why it's a transformer. You have to make that connection from one of the 300-ohm pigtails to the coax shield on the female F-connector for the balun to work properly in this case. Gotcha. I just clicked to these facts a few minutes ago. It's a transformer, not a direct connection. Duh. I'll look for such an adapter next time I am at the parts depot, probably late next week. The shield of the coax will be grounded at some point near the earth ground. At the antenna end of the coax, the shield needs to be connected to the remaining wire on the 300-ohm side which did not get connected to the twinlead. I assuming you are not going to run a seperate ground wire (don't) up to the end of the antenna where the coax connects to the matching transformer. The shield of the coax will be grounded to the earth ground through the grounding block, I thought. I am picturing something with a F/F adapter going through it and bolted to it on either side. This is physically attached to the grounding rod, through an electrically conductive means. The coax comes down from the roof and connects to one side of the F/F adapter with a male F connector, and picks up again on the other side before going to the radio. The shield of the coax is therefore grounded through the grounding rod which is sunk seven feet or more into the earth. That's fine. It's the most common way to ground the shield of the coax. Maybe it's time for a diagram but I think you're close to understanding how to do it. I think I understand, although we are using somewhat different imagery to describe it. Since I am not transmitting I think I will try to hook it up and see what happens. I am at first going to try clipping this to my whip, just to see if I have any signal enhancement. After that I will look at taking it through a tuner and into the external antenna input. Connecting the center wire of the coax to the whip will work but it negates the purpose of using coax as the lead-in conductor. The proper method is to install a connector on the coax which will plug into the external antenna jack on the radio. This way you're connecting the shield of the coax to the radio's electrical ground system which might help to reduce noise. I now understand this, too. I was thinking I would make use of the extra amplification given the whip's input. May still try it, just to see. I've got the adapter for the external antenna input though. It's time to build and see what happens. I'll let you know what happens. It will probably not be until next weekend though. Thanks for all your help. Dave -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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