Ham Radio: Doublet Antennas & Coax

[@de_w8tam]

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

[@timg5tm941]

Glad it works well for you Thomas 73

[@acestudioscouk-Ace-G0ACE]

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. 😃

[@timg5tm941]

Please do! 👍👍

[@johnwest7993]

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'.

[@johnkiljan7441]

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.

[@timg5tm941]

Totally agree. Thank you 73

[@frankwc0o]

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

[@tombaffa9374]

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

[@timg5tm941]

Worth a try 👍👍

[@adyg6ad73]

Interesting information Tim. Congrats on the 6k subs!

[@timg5tm941]

Thanks mate

[@vinjacobo662]

Thanks for this informative video. Cheers wz2j 🎉

[@richardchadwick7152]

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 ...

[@americaswayout4489]

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 !!

[@timg5tm941]

Nice one!

[@dougtaylor7724]

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.

[@christophertitsworth7893]

Great information...thanks for your content.

[@timg5tm941]

Thank you!

[@StreakyP]

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.

[@timg5tm941]

Interesting info thanks

[@nickaxe771]

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

[@timg5tm941]

Mostly if weight was a consideration

[@raydavidsonm0ray.777]

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.

[@timg5tm941]

I’ll need to watch the vid again!

[@raydavidsonm0ray.777]

Thanks Tim, I await your reply in earnest 🙂

[@artc9114]

G5RV or ZS6BKW antennas are not Doublet antenna systems. A true Doublet from any old ARRL antenna handbook is achieved by using a 300ohm, 450ohm ladder line or preferably 600ohm balance line directly terminated to the back of an L or T network type balanced tuner/matchbox, then cutting the antenna legs length to the lowest band of operation. No COAX at all for minimum losses which gives maximum efficiency. A G5RV or ZS6BKW must have at least 50-plus feet of coax to counterbalance the antenna, tuner, and radio.

[@kellypaws]

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.

[@timg5tm941]

I have done some 80m stuff in my small garden .. have a look at my 80m doublet ..

[@wildbill1]

Nice video tim, however the lower you go in the band the less loss, as compared to the higher frequencies

[@timg5tm941]

Good point!

[@davem0udb]

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

[@timg5tm941]

Its all about experimenting Dave!

[@PulsechainProfits]

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?

[@timg5tm941]

For a doublet, if you use a range of bands then the impedance at the balun end of the ladderline will vary massively. A 1:1 is less likely to reduce the impedance to the extent that a tuner may have trouble matching that band. A 9:1 and even a 4:1 may have more trouble in doing so. Some think that because the ladderline is 450 ohm that a 9:1 would change the impedance to 50 ohms - not so!

[@darz3]

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?

[@timg5tm941]

Now that is a very good question

[@ivorbiggen9599]

Very well explained, thank you

[@timg5tm941]

Glad it was helpful!

[@radioguy19510]

Also spend a lot of money on coax connectors buy the best quality connecters that you can find.

[@timg5tm941]

Very wise

[@markg6jvy135]

👍👏👏

[@timg5tm941]

Cheers Mark