Home |
Search |
Today's Posts |
#2
|
|||
|
|||
Ladder Line Entry to Shack
In message , K8JRM
writes 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! Presumably, your ATU has a balanced output for the 450 ohm feeder. If you already have the two (presumably fairly short) pieces of coax lines already in place, I reckon that the first thing you should try is to ground the two shields to the case of the ATU (which I presume is your shack safety ground, and maybe also your RF ground). This should screen the inner conductors from other nearby conductors, and keep the feed balanced. If you don't ground the shields, you might as well not use coax. Obviously, using coax will change the impedance of the line for a short distance, and may affect the settings of the ATU (more on the higher HF bands, of course). But with luck, the ATU will still tune OK. The increase in feeder losses should be negligible. An alternative is to go from the ATU and through the wall using low impedance twin feeder. The spacing between the conductors will be much smaller than the for the 450 ohm feeder, and it should not have much opportunity to become very unbalanced due to the proximity of nearby conductors. It might be advantageous to twist it. I understand that, if the insulating material is suitable, twin mains power cable can be used as twin feeder, especially on the lower HF bands. The current rating of the cable will, of course, depend on what power you intend to run. Ordinary lighting twin flex should handle a couple of hundred watts. But, if you've got a bit of 50 or 75 ohm twin, all the better. -- Ian |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
ladder line?? | Antenna | |||
Using Twin Lead or Ladder Line for your Antenna's Feed-in-Line ? - Then 'consider' a Pair of Vintage Style TV Antenna Clips . . . | Shortwave | |||
Feed Line Length - Ladder Line | Antenna | |||
Source For 72 or 75 Ohm Ladder Line ??? | Antenna |