K7RAW Self-Contained Off-Center-Fed Vertical Dipole (OCFVD)
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- Опубликовано: 25 ноя 2024
- This is the perfect companion antenna for a POTA activation or any field HF operation. It operates 40m-10m and everything in between. The telescopic whip, tent stakes and guy tiedowns are all contained inside the 2" PVC tube. Add a tiny ATU-100 on a short piece of coax and you have a fully-tunable multi-band antenna.
MOTIVATION
This 40-10m compact antenna is intended for very fast setup and take-down with good performance on all bands. Everything you need (except the ATU and coax) is stored inside the 2' long 2" PVC tube.
THEORY
This OCFD vertical has an overall electrical length of about 22 feet which is the size of a typical "flagpole" OCFD. Since the impedance at the 20% feed point is about 450 ohms, a 9:1 UnUn is incorporated along with a good toroid-based common-mode choke. A battery-operated auto-tuner feeds the antenna's BNC connector. The antenna below the feed point is made with 7.5 turns of 1/2" copper foil tape wound around the PVC just to save height. This also keeps the PVC base low to the ground and easy to guy. WIth the ATU connected near the antenna, a 10W RF CW signal will trigger the tuner to tune up to that frequency where the resulting SWR is read out.
ELECTRICAL:
1 5.53m long, 14 section, stainless steel telescopic whip (eBay)
1 4.5' long section of 0.5" to 1.0" wide copper foil tape
1 ATU-100 with internal battery (w/UHF connectors) (eBay)
1 FT-240-31 ferrite core (Amazon)
1 FT-240-43 ferrite core (Amazon)
1 Common-Mode Choke (CMC), wound bi-filar, bi-sectional using 18AWG on Mix 31 core
1 9:1 UnUn, wound tri-filar using 18AWG on Mix 43 core (optional 100pF 3KV cap at input)
1 BNC bulkhead connector (round base, not square)
1 BNC right-angle adapter (to let the short coax jumper connect to the ATU on the ground)
1 3/8" ring terminal crimp style (crimp & solder)
A/R Different colored 18AWG stranded hookup wire (for CMC, TLT and leads)
WINDING DIAGRAMS:
Common-Mode Choke: www.pa9x.com/t... (3rd picture down)
9:1 UnUn TLT: vk6ysf.com/unu... (ignore the page title, it's a misnomer)
PVC (schedule 40):
1 ~24" long 2" tubing (creates the body of the antenna base)
1 2" cap (rounded top) (for the base)
1 2" plug (flat face) (for the reversible whip holder)
1 2" "T" (for mounting the CMC, TLT and BNC cap assembly)
3.25" 3/4" PVC pipe (onto which the CMC & TLT slide)
1 1" cap (rounded top) (holds the BNC bulkhead connector)
1 1" to 3/4" reducer (BNC cap goes over this)
1 2" to 3/4" reducer (mounts the 3/4 tubing supporting the CMC & TLT)
1 set of PVC glue primer and adhesive (to permantly attach all PVC parts)
HARDWARE:
1 3/8-24 threaded stub (~1" long) (to connect inside and outside unions)
2 3/8-24 female unions (one inside plug, one outside)
1 3/8-24 hex bolt (holds the ring terminal for the whip to the inside of the whip union)
3 ~1/2" OD screw eye & nut (for guy cord carabiners to hook to)
3 mini-carabiner clips (for clipping guy cords to the screw eyes)
3 CamEZ tensioners (to tension the guy cords near the tent stakes)
3 tent stakes (place about 2-3 feet from the base of the pipe)
3 3' sections of 3/16" parachute cord (guy cords)
1 2' long piece of 2.5" PVC heat-shrink tubing (goes over the spiral copper foil as protectant)
That’s really cool that it’s all encapsulated like that.
Looks like a nice homebrew antenna for portable operations.
Fine business Sir.
Innovative. You've succeeded in ariusing my curiosity, now off to build it
Nice home brew, well done.
That's amazing, Rudi!!! Can't wait to see the finished product. 73 from W5ODP 😎
I'm posting an update video shortly.
That's awesome! I like how you hid everything inside and used a coil counterpoint. EXCELLENT JOB, SIR!
This is NOT a "counterpoise" but simply the lower 20% of the dipole wrapped a few times around the PVC tubing just to hold it and make it short.
Intriguing design. Thanks for sharing!
What a unique looking antenna (to start with)... but this looks like it would be fun to build and easy to deploy. I'll have to try it (after I learn how to build CMC's and UnUns)!
Watch my TennaTalk #12a and 12b to learn all about it.
Looks interesting indeed! Do you plan a follow up video to show us how you set it up? I'd also like to see how well it performs.
That will be the next video
Considering your design for attic use in my HOA. What are some things you might recommend to enhance operation on 40M? TIA!
I have never thought of doing a coil for the counterpoise(?) ground plane(?) that is epic! How long of a piece of copper did you use? Or is this more a area thing vs length?
It isn't a "counterpoise" so much as the bottom 20% of a short vertical dipole.
@@antennawhisperer-k7raw151 Ah off center fed vertical then. Got it
I use an 18.5' long, 14 section stainless steel telescopic whip plus 4.5' of 1" wide copper foil tape wrapped around the 2' long tube about 7.5 turns. This is NOT a "counterpoise" but simply the lower 20% of the dipole.
Thank you for sharing this with us.KC1TWE.
I like it, but I think the Mad Dog coil is a better design, more compact and lighter. I would imagine this would cost about the same to make as it would cost to buy a commercial version, such as the Mad Dog, Wolf River, Ten Tec or one of the many Chinese made coils that do the same thing once you assemble and obtain all the parts, I doubt this would be that cheap as an antenna, at least here in Australia. I do like the idea of the full kit inside though and the antenna stowage up the guts of the mount, Very clever design.
I'm not sure you understand the distinction here. This is NOT a "vertical" with a loading coil and ground plane. It's an Off-Center-Fed Vertical DIPOLE. It's just fed about 20% (electrically) from the bottom with a 9:1 UnUn and a CMC. Different animal altogether.
@@rudiwiedemann8173 I am aware one is an OCFD the other is base loaded vertical, but essentially they are the same thing. both offer multiple bands on one wire and have a loaded section to deliver said bands. I know they are a different design, but still, they're doing the same thing.
@@bitemykrank1970they are not the “same thing”. A 1/4 wave vertical needs radials. This is an OCFD. There is no loading. The 9:1 unun is for impedance matching and the choke is to prevent common mode currents on the coax. There is no coil to provide inductance. You would use a coil on the 1:4 wave vertical so it can be used on other frequencies.
Got a link for the whip? I have not found one that is not 10mm thread instead of the 3/8 24 that I already have many things for. Even a good source of 3/8 24 to 10mm adaptors would be great! Thanks for open sourcing this!
Go to www.mcmaster.com and search for P/N 92499A793 which is a stainless adapter M10x1.5-F to 3/8-24-M. Cost is about $8
Very nice! Would an antenna like this work better or worse if it were elevated, instead of on the ground?
Ground-mounted OCFDs are just fine with no real advantage to raise them above ground since there is no groundplane.
It's a type of dipole, so the only thing raising it up off the ground might be to lower the launch angle.
Nice work Rudi! deKK7BUC
Hello...I like your design. I have looked inside several 32 foot vertical HF whip antennas in my time. I was a radio guy in the Navy for the better part of 31 years. So I had the privilege of seeing "deconstructed" vertical whips. The twisted strap approach you opted for is okay but not the way that I have seen commercial grade whips made. It should work though. You also need to drill a small hole in the cap end of that PVC pipe to drain off moisture buildup. Moisture will build up in it. Assuming that the cap is pointed down towards the ground right?
Well no, I didn't design this to Navy Mil Standard specs, Sorry, it's for hams not for warfighting. And you can drain any moisture when you un-cap the top that holds the whip.
@@rudiwiedemann8173 I love your designs! Thanks for posting.