View Full Version : Coax->Ladder Line -> Dipole... Balun?
08-31-2009, 02:30 PM
I have been wanting to build a dipole for 20m to compare to my G5RV.
I have a 40 foot section of ladder line, plenty of wire, and insulators. Problem is, I have a tuner with only S)-239s (LDG Z-11 Pro). The coax feed is about 18 feet before exiting the house.
Do I need a balun at the point the Coax switches to Ladder Line?
Thanks for the newbie help!
08-31-2009, 02:55 PM
If you intend building a half-wave dipole for use on only 20m, why not feed it with coax all the way, and do away with the tuner and ladderline? A "coiled-coax" choke at the feedline would be useful. That will produce less loss than the combination of ladderline/balun/coax/tuner that you are proposing.
Here's why: the 40ft of ladderline will be operating with a VSWR of around 5:1 and, assuming it's something like Wireman 551, will have a loss of 0.23dB. The remaining 18ft of coax will be operating with a VSWR of around 16:1 and, assuming something like RG213, will have a loss of 1.37dB. So total feedline loss is 1.6dB to which you have to add the losses in the tuner.
On the other hand, feeding it with 58ft of RG213 produces a loss of just 0.46dB.
09-01-2009, 01:27 PM
I hadn't thought about it that way......I always thought ladder line was much less lossy than coax.
09-01-2009, 04:21 PM
The open wire or twinlead does have lower losses compared to coax for a given length and SWR conditions because in general the currents are lower. The characteristic impedance being 300, 450, etc. means higher voltage but lower current for the same power levels. Lower current means less loss. Remember that the power lost in the conductor is I squared times R so keeping the current down is a good thing .....same thing for electrical power transmission where the voltage is raised way up to lower the current for a given power transmitted. 73, Pete
09-01-2009, 06:33 PM
The open wire or twinlead does have lower losses compared to coax for a given length and SWR conditions because in general the currents are lower.
True - but when feeding an antenna with an impedance close to 50 ohms, the SWR conditions are not the same.
A 20m half-wave dipole at a height of 30ft over average ground will have a feedpoint impedance of 79 ohms at resonance. That represents a VSWR of 1.6:1 on 50 ohm cable, but 9:1 on 450 ohm cable. When feeding this antenna the losses in RG213 would be 0.8dB/100ft, and 0.6dB/100ft if using Wireman 551 ladderline - in other words there's no significant difference.
But the proposed hybrid arrangement is the worst of all worlds - using the ladderline delivers no significant improvement over the 40ft length, but it produces a high VSWR on the remaining 18ft coax section with resultant high losses.
Ladderline all the way, or coax all the way, are much better options than the hybrid arrangement; in this case the marginally higher coax losses could easily be offset by the losses in the tuner needed for ladderline operation.
It's a common mistake to think that ladderline must always produce lower losses; it's not true - you have to analyse a particular system in detail to determine what would be the losses with the different feedline options.
09-02-2009, 04:08 PM
There are lash-ups using a NVIS antenna, 450 Ohm twinlead, a 9:1 balun, and then coax for multiband operation. This system should be analyzed in the same manner. This was sent out to alot of people on official looking MARS letterhead. I'd like to find that article again and analyse it in the same way. Here we go again with the 50 ohm to 450 ohm transition insisting on a 9:1 balun....good luck:) Another case of some one having a lot of luck for one specific case and then declaring it the best thing since sliced bread. 73, Pete