Ladder Line & Metal Objects

Status
Not open for further replies.

radioetc

Member
Joined
Mar 25, 2020
Messages
25
I am going to embark on a project to convert an end fed antenna in an attic fed with 75 ohm coax to a dipole fed with 450 ohm ladder line.

I've been doing some research and it seems that one is supposed to keep metal objects at least a couple inches or more or away from the ladder line in order to not disrupt the balanced characteristics of the line. But I am not certain how important that is in a receive situation.

My goal with the ladder line project is to improve signal gain (dipole vs. end fed) and reduce feed line noise (balanced line vs. TV coax). Also, since the primary listening radio is of a vintage variety with a high impedance balanced input, theoretically I do not need any balun...I can run straight to the radio.

That said, I was going to fish the ladder line wire down a wall that presently has phone wires and CATV coax. It occured to me that maybe I need to remove those wires so as to not interfere with the ladder line. Also, there is a metal box where the wire would come through the wall that I cannot (or do not want to) remove. So that got me thinking about bringing the ladder line from the attic through a closet ceiling instead. I'd rather not poke a hole in the closet ceiling for ladder line but willing to do it if it's the right way to install this. Also, I will have some electrical conduit and telephone wires to navigate around in the attic but I do not expect that to be a big deal. Lastly, it would be ideal if it was permissible to have the ladder line come away from the antenna at a 45 degree angle vs. straight down.

Am I over thinking this for a receive antenna given what I want to accomplish?
 

prcguy

Member
Joined
Jun 30, 2006
Messages
12,484
Location
So Cal - Richardson, TX - Tewksbury, MA
An end fed antenna the same size as a center fed or offset fed will have about the same gain and radiation pattern. If your end fed had a 9:1 balun then I would not expect much improvement changing it to a center fed with ladder line. Except that the ladder line is very sensitive to its surroundings and I would make sure you don't have plaster walls with metal lath under the plaster.

Otherwise you can run the ladder line sideways off the antenna or 45deg angle or anything you want, just keep it away from everything. Does your receiver have a true balanced input for this? If not and one antenna screw goes to chassis ground then the whole thing will be unbalanced. In this case I would run the ladder line to a good 1:1 choke balun and run a very short coax to the radio. The balun, if good enough, will effectively balance the system by virtue of its high level of RF isolation.

I ran a ZS6BKW antenna for several years at home and my office and had to make elevated PVC pipe holders for the ladder line to keep it off all the coax on my home roof and the metal roof at work. The ladder line transferred to coax with a balun on the roof making it easy to run coax to the radios.
 

radioetc

Member
Joined
Mar 25, 2020
Messages
25
Yes, the receiver has a true balanced input. It's an old vintage Hallicrafters from the 30s. Right now, I have a jumper between one of the antenna posts and ground which of course will be removed once ladder line goes in. And the walls are dry wall. Aside from nails and metal corner bead (that's probably not the correct term for that), no metal lathe or anything.

So I gather if I want to run this through the wall where the existing coax and phone wires are, the metal box that origianlly was meant for a telephone jack would not be good, right? This is assuming I pull out the phone wires and coax.

If this was an outside antenna, I'd probably do a balun or unun and coax. In this case the center of the attic antenna run while not immeadatley overhead of the room with the radio, it's pretty close. I estimate I'll need about 35 feet of line. Maybe more depending on what gymnastics I have to go through to route it away from metal things in the attic. I ordered 50 feet so I figure that should be enough for the unexpected.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
12,484
Location
So Cal - Richardson, TX - Tewksbury, MA
If you run the ladder line through a metal box it will interact in some way. If you don't have any fire breaks within the wall you can cut out a hole in the wall, run the ladder line down inside the wall from the attic and install a plastic bracket and plastic blank outlet cover with binding posts for the ladder line connection if you wish. I like to use these, which will mount from the outside of the wall and hold a blank outlet cover. Carlon 1-Gang Non-Metallic Low-Voltage Old Work Bracket-SC100RR - The Home Depot

Yes, the receiver has a true balanced input. It's an old vintage Hallicrafters from the 30s. Right now, I have a jumper between one of the antenna posts and ground which of course will be removed once ladder line goes in. And the walls are dry wall. Aside from nails and metal corner bead (that's probably not the correct term for that), no metal lathe or anything.

So I gather if I want to run this through the wall where the existing coax and phone wires are, the metal box that origianlly was meant for a telephone jack would not be good, right? This is assuming I pull out the phone wires and coax.

If this was an outside antenna, I'd probably do a balun or unun and coax. In this case the center of the attic antenna run while not immeadatley overhead of the room with the radio, it's pretty close. I estimate I'll need about 35 feet of line. Maybe more depending on what gymnastics I have to go through to route it away from metal things in the attic. I ordered 50 feet so I figure that should be enough for the unexpected.
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
6,931
Location
Stockholm, Sweden
The feeder are just as important perfomance wise at receive as it is for transmit. The difference are that if the feeder gets unbalanced and work as an antenna or have SWR it could interfere with other electronic appliances that pick up the RF when you transmit

Wasn't coax invented to get rid of the problem of using insulated stand offs and keeping the ladder line away from conducting objects?

Use as much ladder line as you can in the attic , it's supposed to mellow out the SWR, and then use a balun to connect to coax that you can route however you like. If the receiver only has a higher ohm balanced input then use a balun there as well.

/Ubbe
 

radioetc

Member
Joined
Mar 25, 2020
Messages
25
So Ubbe you are saying that I should not connect the ladder line directly to the radio and instead transistion to coax somewhere between the radio and the attic? This radio has a balanced input but the method for attaching a random wire is short one of the posts to ground and put the wire on the other post. That said, if I transition to coax before the radio, do I need a balun between the coax and the radio or should I just "unbalance" the radio by grounding one of the posts?
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
6,931
Location
Stockholm, Sweden
You'll need to keep ladder line 1 inch away from any objects that could disturb its balance or it will start to operate as an antenna.

If you cannot install the ladder line without it coming in contact with objects or within 1-2 inches of metal, then you'll be better off using coax.

Ladder line are supposed to have a positive effect on SWR when connected to a balanced antenna so try to use as much as possible going from the antenna.

As you have no low impedance unbalanced input you have to use a balun to the radio to convert from unbalanced coax to the radios balanced high impedance antenna input.

It's always interesting, and sometimes fun, to experiment but in this case it might not be worth it if the result will not be much different than what you already have. When space and easy of installation are not on your side then perhaps aim at someting that needs less space and more easy to install, like a small active loop antenna. There's a chinese $50 active loop antenna that seems to work satisfactory to many people. An active amplifier works as a buffer and isolates your radio from the antenna so your radios connection to coax becomes less of a problem.

/Ubbe
 
Status
Not open for further replies.
Top