Ham Radio: Doublet Antennas & Coax

This has been very useful Tim, thanks. I shall be building a doublet this Spring and will bear these points in mind. I'll let you know how it works out. 😃

Your calculation matches my real world experience. I have a 108' doublet into a DG0SA 1:1 balun into 12' of LMR-400 into my tuner. My antenna system works very well. 73 de w8tam

Great information...thanks for your content.

Excellent! I’m going to try it. I’ll only have a run of 3ft of coax

A bit of feed-line terminology: Though some hams call pretty much everything that isn't coax, 'ladder-line', ladder-line is actually composed of 2 wires held apart by separators. The large plastic 450 Ohm 'twin-lead' is 'window-line'.

Interesting information Tim. Congrats on the 6k subs!

Good summary, Tim. I would add that if one is running a short run of thin coax from the tuner to the balun, the heat generated should be a consideration as well. Those lost dBs have to go somewhere, and that is likely to be in heating up the coax. As others have suggested, that could indeed cause its dielectric to melt. If I were doing that with my doublet, I would feel the coax link to the balun from time-to-time to see if it were getting warm --- which is something it might do on one band but not on another.

A better question would be, which is less expensive! Some ladder lines are a dollar a foot.

Would I be correct that you passed some threaded bar through the wall for your ladder line connection, if so how would that affect things?

Tim, how about turning a video over to the particular problems of 80m in a modern (ludicrously small) UK garden size? I have never had insurmountable RF in the shack problem with 40 and upwards. But at 80, the tables turn horribly. About 6 antennas into the problem and it remains a lesser, but detectable problem. I am now considering shares in ferrite beads and in line common mode filters.

Good video, off point, I have some 75-ohm cable hardline at least as large as lm r600, using it would be much better than smaller coax even if the 75/50 mismatch is there for longer runs. thanks !!

Interesting stuff Mike, I’ve been looking at this for a while not sure weather to run open wire feeder round to the front of the house! the next question is should you really use a balun? Seems S match tuners don’t…. keep up the good work I’ll learn mmana one day. M0UDB

Hi Tim, Many thanks for another great & educational video. However I have a 😅for you. What is that program with the Smith Chart in the lower centre called please Tim. It looks looks very useful, but it does not look like a section of "MMANA-GAL" unless I've missed something when I've been using MMANA-GAL when I'm looking at antenna designs? Many Thanks, Kind Regards, Ray, M0RAY.

Some of the RSGB and ARRL antenna handbooks contain charts that show the combinations of doublet top plus balanced feeder lengths to avoid. This helps to avoid extremely reactive loads at the feedpoint, making living with a doublet much easier. The ZS6BKW optimised G5RV is a good approach to the problem making matching easier on most bands. I was taught that using magnetic cored baluns at points of extreme mismatch is bad engineering practice. This can lead to heating of the balun core ( = wasted power) and crossmodulation distortion on TX as the core becomes saturated. A 500 Watt 4:1 balun is rated for its' designed purpose, matching a 50 Ohm coax to a flat 200 Ohm balanced line or a 75 Ohm coax to a 300 Ohm balanced line. Away from these impedances the performance of the balun will be an unknown quantity, so go for the most heavily rated balun you can afford. Better still use balanced line all the way through to a fully balanced tuner, not a T match or L match with a built in balun. As regards the routing of balanced lines, keep any solid objects spaced away from the feeder by at least the distance between the conductors of the feeder and you won't go wrong. Also don't be shy about mixing balanced feeders, for example using 450 Ohm outdoors and running 300 Ohm indoors because it is physically smaller and easier to route. The loss transitioning from 450 to 300 is minute, a fraction of a dB. Indoors it is perfectly acceptabe to use the really cheap 300 Ohm feeder with a solid translucent insulation. This stuff is a problem when used outdoors but is fine indoors because will be kept dry. Its' visual impact is much reduced too when compared to slotted feeder or window line. Keep the videos coming, you are doing a great job. 73 Gareth GW4JPC.

Tim when would you use 300ohm twin feedline rather than 450ohm...or 600ohm for that matter.

Im confused, i just watched another guys video, and he was saying you would need a 9 to 1 balun, or maybe even a 12 to 1. Tim says a 1 to 1. Who is right? Who is wrong?

I run ladderline to the laundry window then a 1to 1 balun and 6 meters of coax to the radio room I can tune it ok with an mfj travel tuner.

300 Ohm for very long Doublet or 9:1 UNUN with low loss RG-8X for very long Random Wire . I use a MFJ 941E tuner with both with very good results ...

Always use at least 7/8 hard line for any run over 30 meters. Unless you have a nice 1500 watt cloud warmer. Then 1/2 Heliax is fine.

There is another one you should look at... the convenience of Co-ax but the lower loss of higher characteristic impedance balanced line.... using both cores of "shotgun" 75 ohm sat tv co-ax (connecting/earthing both braids) gives you 150 ohm balanced line that you don't need to worry about the routing of.