• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

75 ohm vs. 300 ohm.

goodolpg

goodolpg

Just an old fart trying to help.
I have an exterior antenna with about a 40 ft. run of 300 ohm wire.
Would 75ohm be better to use?
I figure I can use converters on both ends to make the change as there is an existing no longer used 75 ohm TV cable running in the same location.
If there is no difference then I won't bother going up on my icy snowy roof.
Thanks for any opinions.
 
Register to hide this ad
You would need to add a balun at the point where the impedance changes. This would be at the antenna if you go to 75 ohm from the antenna. Some baluns are good, but not the Radio Shack ones.

If you only have a 300 ohm input at the tuner, you need another balun to change the impedance back. In this case, run 300 ohm all the way - it saves the insertion loss of the baluns.
 
If you are getting interference would be the only reason I'd run coax. The twin lead is better on long runs because of less loss per foot over coax.


Adding a impedance matching device will add losses and should be avoided it possible along with kinks and wrinkles in the wire.


Barney
 
I was thinking of using the twin lead baluns at both ends..at antenna use twin lead to balun to cable then balun back to twin lead at the receiver.
Receiver has twist tight connections for both 75 and 300 ohm leads.
Based on the comments so far it would seem using 75 ohm would not gain me much?
Reception is "ok" not great at present.
I have a vintage receiver from around 1980.
 
Only two reasons to change to coax I can think of.
1) If you are picking up interference
2) If you can't avoid running right next to metal, like a conduit.This could cause an unpredictable amount of decrease of signal.

If the 300 ohm twinlead is properly spaced away from conductors and your tuner direct input is 300 ohms, putting in the two baluns will only lose some signal.

It would be worthwhile checking if you can figure it out: since your tuner has both inputs, which one goes direct to the first RF stage, and which goes through an internal balun? Then use the setup that has the least total number of baluns, counting the internal one.
 
If you run 300Ω twin lead.From the antenna to the reciever run it twisted. BTW squirrels love to chew up twin lead..
 
Anyone old enough to remember ladder wire? Two bare copper wires separated by evenly spaced plastic spacers? A role of that stuff is huge! Probably not made any more though. :no:
 
Twin lead is a balanced signal; 75ohm is unbalanced. The balanced signal is more reliable over long runs....XLR mics use it for this reason.

Not to hijack the thread....anyone have any suggestions for my crazy idea of connecting two of these antennas so I can place them perpendicular to each other? Being on the ceiling makes adjusting it a pain...and Id like to be able to pull in a few extra stations without having to get out the ladder.

Sent from my SPH-M910 using Tapatalk 2
 
old_tv_nut said:
Only two reasons to change to coax I can think of.
1) If you are picking up interference
2) If you can't avoid running right next to metal, like a conduit.This could cause an unpredictable amount of decrease of signal.

If the 300 ohm twinlead is properly spaced away from conductors and your tuner direct input is 300 ohms, putting in the two baluns will only lose some signal.

It would be worthwhile checking if you can figure it out: since your tuner has both inputs, which one goes direct to the first RF stage, and which goes through an internal balun? Then use the setup that has the least total number of baluns, counting the internal one.
Click to expand...

Almost all tuners use the 75 ohm unbalanced signal going to the RF front end. So the 300 ohm inputs use an internal balun to go to 75 ohm.

I'm not a fan of twinlead, as the impedance changes as it gets close to anything, and getting it close to metal is a the worst of all.

To run it properly, you need to use the proper insulated standoffs for the entire lead length. The common cheap version is also susceptible to insulation cracking and internal corrosion over time when used outside.

If I were you, I'd get the tuner section properly aligned before replacing any cable, provided the cable is not damaged/corroded/poorly installed.
 
It's possible the twin lead runs near some electrical wires in my attic, those would not be in a conduit thought.
I do have the twin lead zip tied to a large-ish lightning rod ground wire at one spot, is that a problem?
Local stations come in strong though a little hissy...no real problems. Just wondered about using the 75 ohm as it is just laying there unused. Both the 75 and 300 ohm ran to seperate antennas mounted on the same pole, one for FM, one for TV.
No longer using the 75 ohm for the TV as I have cable now.
 
goodolpg said:
It's possible the twin lead runs near some electrical wires in my attic, those would not be in a conduit thought.
I do have the twin lead zip tied to a large-ish lightning rod ground wire at one spot, is that a problem?
Local stations come in strong though a little hissy...no real problems. Just wondered about using the 75 ohm as it is just laying there unused. Both the 75 and 300 ohm ran to seperate antennas mounted on the same pole, one for FM, one for TV.
No longer using the 75 ohm for the TV as I have cable now.
Click to expand...

If the 75 ohm ohm cable is sitting there unused, I would try it. Get a decent balun, if possible.
Only one way to find out. I'm not a fan of twin lead at all, I would never use it, except for short runs. The loss in good 75 ohm cable is greatly overstated, in my opinion. As earlier, you can get huge gains by cleaning the gangs and aligning an old receiver, perhaps 10-20dB improvement, versus tiny 0.5dB loss through a cable or balun.

No, the twin lead tied to the ground rod is not going to be optimal...
 
Bob@FM said:
Only one way to find out. I'm not a fan of twin lead at all, I would never use it, except for short runs. The loss in good 75 ohm cable is greatly overstated, in my opinion. As earlier, you can get huge gains by cleaning the gangs and aligning an old receiver, perhaps 10-20dB improvement, versus tiny 0.5dB loss through a cable or balun.

No, the twin lead tied to the ground rod is not going to be optimal...
Click to expand...

I guess that's where my misapprehension about twinlead being lossier than 75 ohm comes from. All else being equal, the 300 ohm may have the edge, but then you lose that advantage if it comes near metal at all...which is why we all used to use standoffs. Coax, you can do anything you want with it without losing signal. Plus, in an outdoor environment it holds up way better than twinlead.
 
Something to consider is that co-ax makes it easy to use an antenna amplifier at the antenna end of the line, which gets rid of any loss problem.
 
Thanks everyone for your help!
I'll try to find a couple good baluns. I won't be going on the roof soon unless the ice and snow happen to melt.
 
You must log in or register to reply here.
Back
Top Bottom