Off Center Fed vertical 5/8 wave??? hows that work

[davev8]

I was researching the Shakespeare big sticks for some unknown reason and as i have been amassing materials to make an aluminum 17M vertical dipole .. a dipole was chosen as no radials are needed and simple construction with the coax going up inside the ally tube feeding the center and no matching devises needed
so this catch my eye when on some forum about 20 years ago someone emailed Shakespeare asking for calefaction on the different models
below is part of the email

>>>>Then in the 90's we designed the first off-center fed 5/8 wave model ABS-1600 made camo green. After a series of wind related internal conductor failures we changed the internal coaxial feed line construction and renamed the product NBS-2010. Bothe th ABS-1600 and the NBS2010 were called Army Big Sticks.<<<<<

Google has come up with nothing...so how would you determine how far up the antenna would be the feed point? Would you be able to find a 50 ohm point .or would there be some form of matching ?? thinking about it just now i suppose a gain master is a OCF 5/8 wave
so if anyone has some thoughts on how the army big stick worked i would be interested

 

[The DB]

I haven't worked with this specific antenna.

As long as each leg is electrically shorter than an electrical half wavelength, it will act just like a monopole vertical antenna.


The DB

 

[davev8]

I haven't worked with this specific antenna.

As long as each leg is electrically shorter than an electrical half wavelength, it will act just like a monopole vertical antenna.


The DB

i understand that but if i was to design a vertical OCF 5/8th wave how would i determine the feed point?

 

[Holydvr]

Likely with a nano vna or an antenna analyzer.

 

[Crawdad]

Sounds like a challenging project. You would most certainly want an analyzer to tune that one with. Hope it works out for you.

How high AGL will the bottom element be?

73

 

[Dr. Frankenstein]

I feel like it's finally time to post...........
Read this "https://hamcall.net/7bandocf.html".
I've had mine for 8 years, it covers 6-80 meters... 35 feet high at the center and 12 feet high at each end. I do use an antenna tuner with it when I need it for 75 meters, and run the legal limit into it.
Will not work on 11 meters due to design, and mine being a total of 135 feet long will not talk 160 meters. Sure they are expensive, but they last and are well designed.

73

 

[davev8]

not really sure why you posted this here ...i am asking about the design of a vertical antenna
but looks like a good antenna and deserves to be posted someplace

 

[davev8]

yes, an analyzer will be needed ..but i had an idea... i could design it for any frequency and scale it up or down ...so if i had 2 telescopic antennae, you can buy pretty big ones or have some ally tubing with a lot of adjustment and set the length for 5/8 wave for whatever frequency, feed it in the center and then say shorten the bottom element and then lengthen the top element by the same increments that would keep it 5/8 wave but would move the feed point down taking readings as you go if i made this and put on my tower the AGL would be 55 feet but don't have the time at the moment my next antenna build is a vertical CFD for 17 meters which i have amassed the materials for it and for a center feed ally antenna there is no R&D

 

[BJ radionut]

K4TR Antenna Mfg and Sales

https://kk4tr.tripod.com/kk4tr_windom.jpg

 

[Dr_DX]

This is from Tech833, the Copper Talk forum Moderator that did all of the fiberglass antenna expose's referring to the NBS2010 :

Posted on Sunday, June 25, 2006 - 10:59 am:​

​

---

It is not 5/8. It is a 3/8 wave element over a 1/4 wave sleeve. However, the total combined elements length does measure 5/8 wave.

The regular 'Big Stick' (2 section antenna) was a 1/4 wave element over a 1/4 wave sleeve.

 

[Dr_DX]

More NBS2010 info from Copper Talk (typos included):

Here is the info as requested. The answers are direct from Shakespeare:-

Q. What is the differnce between the Camo Army Bigstick ABS16020 and the white NBS2010 other than colour?

A.Only difference beside color was an improvement in how we manufactured the "feedline" inside the lower section. This feedline was astep linear trasformer made of 1/16 wave length "Q" sections for matching the antenna across the bandwidth. We had some discontinuity problems on the 1600. So when we improved the design we changed the color

Q.What wave length is the antenna?

A.The antenna is a unique application of a 5/8 wave radiator. It is an off-center fed 5/8 wave coaxial sleeve antenna. We have a VHF version of it (136-174 MHz ) that is patented. We simply adapted the design to 27 MHz. The feed point of the antenna ie where the RF is acutually imparted to the antenna is at the top of the base stick so there is a 1/4 wave below and 3/8 wave above the feed point. The coaxial sleeve in the base section is why mo radials are needed and why it is not necesasary to cut the coax to a specific length.

Q.What about grounding the antenna?

A.Don't worry about "grounding the antenna"
But do use the inline static arrestor and keep it close to the ground with a good ground rod......this will keep the static buildup "charge under control and will "probably keep the lightening at bay too.

Specs
Technical Data -
ABS-1600 - Color Camo green
NBS2010 Color White
Power 4000 W PEP ICAS
Bandwidth 2.5 MHz (26-28.5 MHz)
Performance 5/8-wave
Wind rating 110 MPH
Length 21' nominal
Sections 3-UPS shippable
Mounting 2 U-bolts, supplied


ABS-1620 -
Color white
Power 1000 W PEP ICAS
Bandwidth 2.5 MHz (26-28.5 MHz)
Performance 1/2-wave
Wind rating 110 MPH
Length 18' nominal
Sections 2-UPS shippable
Electrical EQ Linear transformer matched 1/2-wave
Mounting 2 U-bolts supplied