The Universal HF Antenna

Intro

I was interested in it for quite some time and now got myself a Home and amateur (ham) radio license.

This allows me to build radios, antennas and other equipment as well as to use it on the air. The first radio I build was a kit, a QDX - Digital Transceiver. This transceiver allowed me to work multiple bands, 80m, 40m, 30m and 20m.

I knew from the documentation a bit about the propagation, pros and cons, but could not decide which ones I would really like to dig into deeper. So I decided that I want a fairly broadband antenna to send at least in the bands which the QDX supports and listen to the whole HF spectrum.

I had no shack jet and wanted to use the antenna portable as well as from home, by just trowing some coax out of the window and up onto my roof terrace.

Interesting Antenna Types and why I did no chose them

End Fed Half Wave

An end fed half-wave (EFHW) is, as the name suggests, an antenna that is fed at one end and has the length of one half of the desired wave length. This type of antenna is quite popular for stationary and portable use, for its simplicity and easy deployment. You just attach one end to your radio and bring the other one somewhere else, the best case: in the air.

EFHW

For me the main disadvantage is, that you need to adjust the wire length to the band you want to use. In the meantime I learned that there are relatively simple winch setups using fishing wheels to wind in and out wire to set the perfect length, but it’s still a bit to unpractical for my use case.

Hand-adjustable Antennas

There are different antennas where you can change the resonant frequency by shortening the antenna or adjusting coils and capacitors. These setups are not ideal for me, for they are only practical when you are operating next to the antenna.

Chameleon MPAS 2.0 (Modular Portable Antenna System)

Then I stumbled across the Chameleon MPAS 2.0 and was amazed by such a versatile set to build many antennas. Unfortunately the price here with ~750 EUR was a bit high and quite unreasonable for my 69 EUR radio.

So I tried to find out how this antenna works. It is a so-called end fed random wire antenna (EFRW) and has the strong advantage over the EFHW that you don’t have to match the wave length of the band you use, but you can choose a length that works some-what good on many bands.

But why is this antenna so special and better than other versions of random wire antennas? The build quality and available accessories are definitely a factor, but the main reason must be the matchin unit.

Many designs use 9:1 transformers wound on ring ferrite, like the OM0ET | Mini 9:1. But this design utilizes a 5:1 broadband UnUn would on a binocular core, quite similar to what I used in my QDX radio to drive the output power.

I decided to search for resources and to build a similar transformer.

My Version - DM8TE Random Wire

I found out that the larger variants use 6 ferrite rings on two copper pipes. The smaller variant uses a binocular ferrite core with small copper tubes as primary winding.

I really liked the idea to use copper pipe, for it will have a low resistance during operation, but also the electrical DV resistance between the antenna and ground will be minimal. This prevents static electricity from building up, reducing possible noise and protecting the equipment.

For my build I wanted to get a versatile antenna systems, which I can use to build all kinds of portable and semi stational antennas, not just the UnUn.

Transformer Build

Material transformer

  • Binocular core AMIDON BN43-7051
  • 6.35 mm copper tubing from ebay
  • AWG16 PTFE isolated silver plated wire from DX-wire
  • Coax connector, I build variants with BNC or N-Type

5:1

  1. I straitened the copper tubing as good as I could with a hammer and pliers. Then I used a file to remove a bit material to insert it in the two holes. It is important to debur all edges, to prevent damage to the wire isolation.

5:1

  1. On top, I used strains of copper wire and a lot of solder to make a low resistance connection between the pipes. Smaller and lighter is always nice, but having a bit more good conducting thermal mass on the ferrite is even better. This helps to dissipate heat if you overload the transformer for short periods of time.
  2. The silver wire is soldered to the bottom of left tube and then the turns begin on the right one next to it. After 5 turns the cable leaves on the upper left tube.
  3. Make sure to leave a loop on the bottom left, just before the final exit. I used a cable tie to prevent the loop from slipping into the pipe in the future.

5:1

  1. Make the following connections
    1. PTFE wire to the radiator terminal
    2. Left bottom tube to the coax core
    3. Right bottom tube to the coax shield

5:1

Case

For the case there are many options, small water resistend boxes are a good one. I opted to use PVC pressure pipe, for its rigidity and the possibility to attach different accessories directly to a T-pipe.

I wanted to be able to push the antenna system in the ground, attach the desired radiator and start communicating.

Material

I drilled holes in all 3 end-caps and attached the following

  • M8 bolt + washer + cap + flange nut + washer + wing nut
  • BNC or N-Type connector
  • 3/8-24 cupping nut

I soldered the UnUn transformer in and glued 2 of the 3 caps. The third cap, holding the BNC connector, is only held and sealed via electrical tape. This is done to keep the device serviceable.

UnUn Case

Radiators / Accessories

Usage

First you have to choose a radiator and if applicable radials.

The radiator can be chosen from a wide range, but it can’t be fully random! You need to avoid the wavelength of the band you want to work, as well as multiples of those!

Here is a more in depth explanation, tables as well as code to calculate usable length, https://udel.edu/~mm/ham/randomWire/.

These are some length I noted, the highlighted ones were already tested.

  • ~3 m (~10 ft)
  • 7.5 m (~25 ft)
  • 11 m (~35 ft)
  • 18 m (~60 ft)
  • 22 m (~75 ft)
  • 26 m (~85 ft)
  • 41 m (~135 ft)

With a large radiator there are some frequencies you can work without a tuner. I do not recommend this, for the SWR can change drastically through small environmental changes, like rain or wind changing the radiators shape!

I had great results with a simple manual tuner, the Open Source ATU 100 (7x7) as well as the internal tuner in the FTdx10.

~3 m Foldable Radiator / Whip

If you have nothing else this can be usable. I attached the whip to my backpack and when using a 7.5 m counterpoise I could make some contacts.

Whip

7.5 m

I attached simple 1.5 mm² PVC isolated installation cable to a 7 m fishing rod and brought it up a few meters with my Geroh Kurbelmast, it worked amazing! I had great success on 60, 40, 30 and 20 m and worked VK and many US states from my QTH in Germany. However, higher frequencies like 12 m where nearly dead to me.

18 m

Here I used the small 0.5 mm² wire, and it worked great as well. I mainly tried it portable.

22 m Delta Loop

I used a PVC Y-piece to build a holder for two 8 m fishing rods. The cable was fed through the rods and attached to the UnUn. This antenna worked great until one of the fishing rods god storm damage and I wanted to try something new.

Delta Loop

26 m Weird L

This is my current configuration. I have the 7.5 m vertical and un top 18.5 m wire is connected, going down in my garden to a 6 m high fishing rod. I really like how great 12 m band works with this, compared to just the 7.5 m. When a storm comes, I wind down the mast, but the antenna is still usable.