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Ladder Line Entry to Shack
On Mar 14, 6:05*pm, "Peter" wrote:
"K8JRM" wrote in message ... Hi Guys, This is an age old question - what is the best way to get 450 Ohm ladder line into my shack antenna tuning unit (ATU)? *Here is my situation; I have two 4" PVC tubes inserted into the exterior wall that allow the entrance of coaxial cable into my shack. *When I run the ladder line into the shack through one of these ports, the multi-band wire antenna fed with the ladder line goes wild! *I can not get the antenna to tune with any of my ATUs. I have received several suggestions on how I can overcome this problem. The most promising one seems to be to use two short pieces of coax with center conductor connected to either side of the ladder line. These pieces of coax, in turn, would be fed through the PVC hole into the shack and the center conductors connected to the ATU. *The shield on the coax would float with no connection to any thing. *This solution sounds good; however, common sense tells me that ladder feed line will still suffer because of its proximity to other objects. Any thoughts on this solution? Another possibility is using a piece of Plexiglas place under a metal framed window pretty much as shown in the old hand books. *My problem with this solution is that the window is on the other side of the room from operating table, the two coax entry ports and AC power. Short of placing the tuner outside in the elements, is there a solution to my problem? *I would be interested in hear what has been tried! Tom, K8JRM in OKC Howdy Tom The two short pieces of coax with center conductor connected to either side of the ladder line is perhaps the best solution. It was covered in QST/The Doctor is In, June 2008. In the article it suggests using the best quality coax on hand, keeping the run to the absolute minimum and the coax shields should be tired at both ends and connected to the station RF earth. This configuration introduces a small section 100ohm nominal impedance transmission line into the antenna system which should have minimal losses, but most important is that it is still part of *the balance transmission line. The losses for say RG213 at 4mtr would be almost nothing on 80mtr and perhaps up to 2db on 10mtr. Obviously the lengths of coax need to be exactly the same length. If you would like a copy of the article drop me email. Peter *VK6YSF http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm Hi, Peter. I read the QST article and thought it was crazy and so I tried replacing the 5 ft length of ladder line from my antenna tuner up the wall to the two feed through insulators going through the wall to the 600 ohm feed line outside. With the 5 ft ladder line I could tune the antenna, a 160 meter lazy quad loop, to all bands through 15 meters. With the 5 ft section of double coax, just like the doctor said, the antenna would not tune for 20 meters or any higher band. I put the old ladder line section back. Remember what all the discussions say about keeping the feed line away from metal objects? Here we are introducing a 5 ft length of metal just 1/4 inch from the feed line. The extra capacitance just kills the feed line. Yes, I was using RG-213. So, the doctor in nuts, as far as I can tell. Paul, KD7HB |
#2
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Ladder Line Entry to Shack
Howdy Tom The two short pieces of coax with center conductor connected to either side of the ladder line is perhaps the best solution. It was covered in QST/The Doctor is In, June 2008. In the article it suggests using the best quality coax on hand, keeping the run to the absolute minimum and the coax shields should be tired at both ends and connected to the station RF earth. This configuration introduces a small section 100ohm nominal impedance transmission line into the antenna system which should have minimal losses, but most important is that it is still part of the balance transmission line. The losses for say RG213 at 4mtr would be almost nothing on 80mtr and perhaps up to 2db on 10mtr. Obviously the lengths of coax need to be exactly the same length. If you would like a copy of the article drop me email. Peter VK6YSF http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm Hi, Peter. I read the QST article and thought it was crazy and so I tried replacing the 5 ft length of ladder line from my antenna tuner up the wall to the two feed through insulators going through the wall to the 600 ohm feed line outside. With the 5 ft ladder line I could tune the antenna, a 160 meter lazy quad loop, to all bands through 15 meters. With the 5 ft section of double coax, just like the doctor said, the antenna would not tune for 20 meters or any higher band. I put the old ladder line section back. Remember what all the discussions say about keeping the feed line away from metal objects? Here we are introducing a 5 ft length of metal just 1/4 inch from the feed line. The extra capacitance just kills the feed line. Yes, I was using RG-213. So, the doctor in nuts, as far as I can tell. Paul, KD7HB Paul That's interesting and noted as I was intending to use that approach to get the balanced line into my new shack, which is a metal shed. I guess it may be back the old drawing board. Any suggestions? Peter VK6YSF http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm |
#3
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Ladder Line Entry to Shack
" wrote in
: .... Remember what all the discussions say about keeping the feed line away from metal objects? Here we are introducing a 5 ft length of metal just 1/4 inch from the feed line. The extra capacitance just kills the feed line. Yes, I was using RG-213. Paul, The "5 ft length of metal just 1/4 inch from the feed line" is not a good method of analysing the behaviour. The treatment of the shield ends is very important to the operation of these things. If the shield ends at each end are tied together, and each end effectively grounded (and that is a big ask), then any common mode current on the incoming ladder line flows to ground via the outboard ground connection, and the section of dual coax transforms the impedance seen looking into the ladder line in a quite predictable way, and with some loss. The transformed impedance might not be within the range of your tuner. If the outboard shield ends are not effectively grounded, then the configuration does essentially nothing to reduce common mode current entering the shack, so the system is a fails it its primary objective. Owen |
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