Other SOTA sites: SOTAwatch | SOTA Home | Database | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

Coaxial dipole


#1

Wanted to build the simplest to carry/deploy antenna I could think of as I try out SOTA operating.

I used 25’ of RG174, feeding 16.5’ of 22AWG wire with the center conductor. I then placed an RF choke (7 turns, type 31) on the coax 16.5’ from the feedpoint (where the RG174 meets the 22AWG wire).

The outside of the coax shield (up to the choke) serves as the other leg of the dipole, so it is in effect a standard center-fed halfwave dipole on 20m, except that the feedline runs inside of one of the dipole legs. So I just connect rig at one end, and lay the antenna over whatever shrubs/trees are available. Could also pull it up over a taller tree with a single line.

From a deployment/mechanical standpoint similar to an end-fed, but doesn’t rely on a matching transformer or need any counterpoise. Don’t see an easy way to make it multibanded though, which is a drawback.

But seems to “work” well enough for four activations now…

73,
N1VF


#2

Ben,

Your antenna idea is good, and you had an easy-copy 20M signal to Colorado today when you were on W6/CC-072 (San Bruno Mtn).

If you only activate on one band, it’s fine. I started out with a simple dipole for 20M, and it worked well on that band, but I quickly realized that I wanted to operate efficiently on several bands.

These days I often carry two antennas - both are end-fed. Both systems work well on many bands, and the total weight of both wires is only a few ounces. The antenna I use depends mostly on the space and supports available on the summit.

Wire 1: 66 feet of stranded #24 teflon wire, with a link at about 52 feet. No counterpoise.

A) EFHW on 40M - no counterpoise needed
B) EFFW on 20M - no CP
C) 3/2 Wave on 15M - no CP
D) Open the link at 52 feet - it’s an EFFW on 17M - no CP
E) Open the link at 52 feet - it’s close to EFHW on 30M - easy with my tuner - no CP
F) The entire 66 feet also tunes up nicely on 30M with my tuner, high-Z, reactive, no CP

Wire 2: 52 feet of stranded #24 teflon wire, with links at about 46 feet and 33 feet.
A short, convenient 12-foot #24 wire is used as a counterpoise with this antenna

A) Medium-Z resonant feed on 40M with CP
B) Low-Z resonant feed on 20M with CP
C) EFHW on 30M - high-Z - no CP needed
D) Open link at 46 feet - EFFW on 15M - high-Z, no CP needed
E) Open link at 33 feet - EFHW for 20M, high-Z, no CP needed

Many other useful band-tuning combinations are available with both antennas.

Over the years I’ve built and used many small tuners. My current favorite has taps and switches so I can efficiently match a variety of frequencies and impedances. The basic ideas are:

  1. T106-6 powdered iron toroid core for best efficiency
  2. Separate primary and secondary with taps on both
  3. Unbalanced
  4. About 6-22 MHz
  5. Two polyvaricon caps for input and output tuning
  6. Dan Tayloe-type LED bridge for perfect nulls
  7. 50 ohm input
  8. Micro toggle switches for added C and L, tap selection
  9. Multiple impedance outputs to mini-banana jacks
  10. Matching range covers about 10 to 5000+ ohms resistive, plus a wide range of reactive matches - but not the whole Smith chart
  11. 5 ounces
  12. 4" X 2" X 1.5" - plastic case, aluminum panel
  13. 2 feet of RG-316 coax to the radio
  14. Handles 10W CW
  15. I have a smaller version that weighs 3-1/2 ounces
  16. T68-6 toroid
  17. 3" X 2" X 1.5"

Using a tuner gives you maximum flexibility to cover various bands and use a variety of wires and counterpoises, while always getting full power and best efficiency from your radio. Tuners also provide added selectivity to permit operating along-side of other SOTA operators on different bands at the same time.

Once you “learn” your tuner and your wire, you can jump from band to band in a few seconds. You also can use an autotuner, or combine an autotuner with a manual tuner or step-up transformer designed to match high impedances. The idea is to get maximum performance from a few ounces of gear, while avoiding the inevitable losses of small coax cable.

It is important to get your wire as high as possible. While many contacts can be made with a low wire, better and easier contacts result when the wire is raised. I use an inexpensive telescoping fishing pole that gets my wire up to about 18 feet. I try to have the far end as high as possible, but on some alpine peaks, the far end may be only a couple of feet high.

In practice I see little difference in on-air performance between my two systems - usually I operate 40-30-20-17 meter CW. I look at my RBN spots afterwards. Over time any large differences would be apparent. Even some of the reactive matches I use seem to work really well, even without a counterpoise.

Please experiment, try various wires and tuning systems, always with the goal of getting more of your signal to radiate. The more bands you cover, the more chasers will follow!

If they hear you well, they will be waiting for you!

73 and CUL,

George
KX0R


#3

Hi Ben,
Great to hear about the coaxial dipole - another option could be a J-pole (covered in another thread on this reflector). Easy set up, no radials and small and light to carry. I actually made one for 20, 17 and 15 meters.

73 Ed.


#4

Not sure if you’ve read ae6ty.com. The site has a design for a resonant feedline antenna (RFL) same idea as a coaxial dipole.

http://ae6ty.com/Papers_files/Refining%20an%20End%20Fed%20Vertical%20Dipole.pdf

72 Mela JI1KBF


#5

Just a note about using a 1L or 3L/2 wire as an EFHW - the radiation pattern is no longer two main broadside lobes. The radiation pattern becomes four-lobed (1L) or six-lobed (3L/2) with nulls broadside.

Barry N1EU


#6

Hi,
I used a Fuchs-Antenna until now. Because I made them for 40m and up. 2017 I used them most time in EA8 for SOTA on several Bands (with KX3 and the PFR-3) - success.
This year (2018) I took with me the HB1-B and the Bazooka for 20m. - also full success.
see here Bazooka: http://www.hb9fih.org/?p=p_446&sName=ea8--2017-12-2018-bazooka-antenne

Bazooka:
When I them buit up 11/2017 in my QTH HS (Koh Samui) I suddently had QSO with Japan and China.
Also recognized when I compared against a Windom: the Bazooka collects less QRM from the PowerLines near the street.

If you have an antenna and worked with them sucessfull, then you know all about your antenna.
This speeds up your time to become QRV on the top.
I don’t look for any others, my requirement is covered with them.

HB9FIH
73 de Erich


#7

I can testify to George’s antennas performance I have chased/worked him on his SOTA expeditions a few times over the last five years. Another end fed random wire I had some success with is an 81 foot radiator and 25 foot counterpoise run underneath the antenna fine tuned with the kx3 or a You Kits tuner with the ft 817. Works quite well on 80m up to 150km if you need to work some more local chasers you can’t get on 40m .
Great past time living the dream with antenna experiments.
Ian vk5cz …