DOUBLET ANTENNA to use on 40 Metres (HOW SHORT CAN YOU MAKE ONE?)
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- Опубликовано: 29 сен 2024
- Like me, lots of operators have looked to squeeze in an antenna for 40 metres, plus other bands into their small garden or yard. The Doublet Antenna provides a great multi-band option. But just how short can we make one to still be efficient on 40 metres?
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A half wave on 40m is close to a quarter wave on 80m. A 40m dipole may not work on 80m, but is possible with a doublet
I am able to tune my 144 foot doublet to 160M so a half wave is not needed. I use an MFJ-993B Intellituner with it.
Yep can be done. With a compromise.
The difference with the g5rv design is that the length of the feedline and radiating eliment is such that the voltage, ampres,reactance are relatively low on the frequencies its usuable on keeping losses to a minimum, whereas taking the ladder line all way to the shack may have higher voltage, ampes, reactance at certain lengths of the feedline which may tax the system more
Small garden antennas applies to allot of operators, so this is great information Tim, Happy New Year to you and your Family
Thank you Robbie and the same to you and yours my friend 73
Extremely interesting Tim, I really enjoy these videos!
Thanks Walt!!
Im working on a 13 m long aluminium tube rotating dipole . with remote ATU at the centre. The tube reduces reactance over copper wire and also increases antenna eff to about 95%. Similar idea as a mag loop with larger tubing.
I will need to switch in a couple high Q inductors and some extra capacitance to cover 80m. The remote Atu has 10uH and about 880pf. 3.5Mhz needs around 55uH Radiation resistance at 3.5Mhz is just below 5ohms. I will need to keep total R of tuner and switched L/C to under about 4ohm to achieve goal of 50% System efficiency on 80m..Also needs to be at least 16m high to reduce ground losses on 80m. long term project in design phase. Will also like to be able to make the thing go from horizontal to vertical dipole somehow. ATU losses will be tested with reactive dummy load simulating feedpoint impedances,in a calorimeter. Lot more work than my EFHW!
Sounds a good project!
Great stuff again Tim. As you know my end game is a 66ft doublet as that all I can fit in here. I will use it on 80m but any antenna is better than no antenna.
Totally agree!
Well presented Tim, Im with you on max shortening. The doublet is a brilliant antenna. Sadly I cant stand the ladderline being launched back n fore against the house here in the almost constant gales lol
Fair enough - thank goodness for the EFHW!
Hello Tim.
I am using 100 ohm twin feeder from Spectrum Communications it is very thin and isn’t bothered by electrical wiring or metal .
Mine comes in to my loft across all the light wires and down my stoothing wall with all my other coaxial cables.
I can run 400 watts without any problem.
Regards Dave M0IOK.
Thanks for the info! 73
Hi Tim, I have a 112 foot Doublet fed with 300 ohm feeder using a 1:1 current balun in the shack and about 1 foot of RG8X to my Palstar AT2K tuner, and it it seems to work well on 80mtrs
Yes that would be just over 80% the length of a half wave on 3.5 MHz so would be a rough comparison with a 53ft version for 40m. Pretty decent antenna. 73
I’ve put up a 15.2m (~46ft) doublet with 12m ladder line and found v high swr, as you described. It is also worth thinking about where the lost power is being dissipated , my tuner died for 40m but carried on working at other frequencies… It might be a bad idea to run 100 W through a tuner dealing with such loads, particularly digital modes with high duty cycle and low peak to average power ratios.
Indeed - think of the voltages! A lot of auto-ATU's can easily be fried when forcing shorter antennas on to lower frequencies. Been there and done that!! 73
Would be nice to see the comparison of a regular Doublet, dipole, compared to a linear loaded dipole with ladder line legs, compared to the linear loaded doublet dipole with the third wire, and show how they progressively get shorter and shorter. 66', 53', and 33' respectively
I have a three-wire linear loaded doublet and the legs are 16 and 1/2 ft on each side and I'm able to tune up 80 m with an ATU
Hmmm interesting
Rather than adding an additional wire to 450 ohm ladder line (woven between the windows) would you get the same effect By having the third wire come off of the feed point parallel to the 450 ohm ladder line but not touching or intertwining? Should this wire need to be the same length as the 450 or does it need to be shorter or longer? It would be so much easier to do it this way than having to interweave it and tape it to the ladder line that is very time consuming!
Only way to tell this is to experiment I think Pat?
Now I know why a doublet didn't work for me here. Plus I can't get open wire or ladder line back to the shack.
Ah I see
Really enjoying the doublet series of videos Tim, well done. Looking forward to that TLD walkthrough if ya do one
Thank you! That will come soon
What happens if you introduce loading coils?
You CAN do this to bring in a lower band but this may inhibit pattern/radiation on higher bands?
Good stuff as usual, thanks Tim. Do you have any experience or ladder line fed, fan dipoles please? Meantime, I'll go and check your video list. 🙂👍
I can’t remember! LOL I am getting old
@@timg5tm941 I know that feeling being quite few years older than you!
For me I have included A third wire woven in and out of the windows of the 450 ohm ladder line 17 ft legs on each side and using a 4:1 balun. Works great from 80 to 10 m
Nice!!
@@timg5tm941 Wow!!!! That is real? Do you use a current-type 4:1 balun? Pse. more information about this !!!!
Great analysis, Tim. I always enjoy your video.
Thank you Gary much appreciated 73
I made an 80m ocf dipole feed with coax via a homemade 6:1 balun, the longer side doubled back on itself with only 6m between the 2 parts of the longest leg. I was able to work ZL on 40m with 50Watts
Yes indeed, the Balun did a lot of the matching and the antenna was a full size half wave. Great contact 👍
@@timg5tm941 I was told that the antenna wouldn't work in that configuration and I was wasting my time
@@ianbardell8030 as an ocfd it should be ok on even harmonics. So 40 would be ok.
Hi Tim, Happy New year
Great video (another one) using my other account now.
Well smarty pants...... What would you do with 5w(FT817) and a 8x8m garden?
🤔😁
Motters M7TRS 73 👍🏻
Great question .. maybe a video in this one!! Which bands?
@@timg5tm941 am gready as many as possible. I mainly use 20/40 and 2m.
Ive tried a vertical, dipole and efhw. Not much space with children so my setup is a pop up kinda use. My main radio use is portable sota but I like to have the option at home from time to time. No trees and I am a mid terraced house and same to the back kind of setup.
@@m7trsradio would a no radials vertical be an option?
@@timg5tm941 yeah I will try anything. I have two mast in my armoury a 8 well 7m carp pole and a 10m DX commander classic pole would love the DX commander expedition pole for sota.
I use a shorty 40 end fed (40-6m) attached to the top of a 2m wooden fence around my 7x7m garden. With 5 watts it hasn't a great signal but is a lot of fun on FT8. Made contacts with most of the states here in the USA and most countries in Europe.
Another interesting video .
Cheers mate
Great info
Thanks!