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Station Antennas

Home-brew Fan Dipole - 20/17/15/10m

Dipole shown when configured for 10/15/17 & 20m

I design and construct my main station antennas, making frequent modifications to help improve performance and develop my technical skills.

The antenna is currently configured as two independently centre-fed half wave inverted V dipoles for 10m. The elements are configured in a cross, and switchable from the shack to obtain maximum gain from their directivity.

Past configurations have included various fan dipoles covering 20, 17, 15 & 10m (shown left), and flat-tops for 10m.

I generally run the dipoles cut resonant so no ATU is required for the main bands. For other bands I rely on an ATU for a match, but due to the increased mis-matched coaxial feeder loss when operating at those frequencies, this is not the primary mode of operation.


Antenna Construction

Balun feeding all antenna elements

As with all dipoles this antenna is very simple, cheap and easy to construct. I use stranded (pre-stretched) copper wire for the antenna elements (3mm diameter), terminated at each end with ceramic insulators.

When in fan configuration, elements are separated by a number of wooden dowel verticals to reduce unwanted interactions (giving a separation of ~ 180mm).

For the dipole centre I opted for a combined 1:1 balun (see right - showing the multiple connections), connecting the copper antenna cable (balanced) and the coaxial feeder (unbalanced).

This allows the antenna to be driven using a single feeder whilst remaining resonant on each band.

A cheaper alternative would be to use a simple dipole centre and coil ~6m coax to form an ugly balun.

Component List

Antenna:

  • Watson Flexweave multi-stranded copper wire (3mm diameter)
  • 1:1 balun/dipole centre (use two of these when dual feeding)
  • Ceramic dog-bone insulators
  • 6mm wooden dowel (for element spacers)

Support Pole:

  • 3m aluminium pole + 2.5m wooden pole + 2 * 1.5m wooden poles
  • 4 x large aluminium jubilee clips (to attach the upper two poles together)
  • 4 way hook guy clamp
  • 30m nylon braided cord (5mm) for guys
  • 4 x cable grips (to fix guys to supports)
  • 4 x masonry eyed rawl-plugs (6mm)

Physical Installation

FreeCAD model of antenna and surrounding roof area

The antenna is mounted on the roof of a three storey house, with the base of the mounting pole approximately 10m off the ground. The dipole centre is a further 6m higher, with the antenna elements running at an angle of ~90 degrees between elements as shown in the diagram to the left.

The mast itself is a mixture of a two 1.5m wooden poles (bottom), one 2.5m wooden pole (middle) and one 3m aluminium pole (top), strapped together with jubilee clips/screws (see top left diagram to the left).

The total mast stands at a height of 6m from the roof. Four nylon guy cables are strung out and fastened to the brick wall via masonry eyed rawl-plugs.

The roof itself is in a 'V' shape, with a boxed section rising out of one side. For 10m operation there are no obstacles within half a wave length of the dipole centre, but lower bands will see some near field interactions with the surrounding roof structure.

Fan dipole configured for 10/15/17 & 20M

Antenna Feeder

I use coaxial feeders for all bands routed from the shack via a vent in the ceiling to the roof (see below). I'm presently using two runs (12m and 16m) of Mini-RG8 (RG8X) 50 Ohm coax cable. I operate the dipole resonant on the bands required, so am only concerned with matched feeder loss, being acceptable on these short runs.

Based on my current Station Equipment, with a 16m feeder run and peak output power of 100w, the below table gives estimates of expected matched feeder losses.

Coax feeders from roof

Band Loss (dB/m) Loss (dB/16m) Total Loss (@100w)
20m 0.040 dB 0.635 dB 14w
17m 0.045 dB 0.72 dB 15w
15m 0.049 dB 0.787 dB 17w
10m 0.057 dB 0.913 dB 19w

Antenna Patterns

The below diagrams show the elevation radiation patterns for both the flat-top half-wave dipole for 10M (left) and the same antenna but in inverted-V configuration.

Both plots show the pattern for the antenna at 16M (total height from real ground - black line) and 6M (height from roof - blue line) - the real radiation pattern will be somewhere in the middle (nearer the 16M plot), depending on interactions with the surrounding roof structure etc.

Elevation radiation pattern for flat top at 16M (black) and 6M (blue)

Elevation radiation pattern for inverted-V at 16M (black) and 6M (blue)


As can be seen, at a height of 16M there isn't a huge difference between the flat-top and inverted-V. However, when operating with dual independently fed inverted V's (pointing in different directions), the difference in signal strength is observable on some signals (up to one or two S points).

Elevation radiation pattern at 90 degrees for inverted-V at 16M (black) and 6M (blue)

Dual independently fed crossed 10M inverted-V dipoles


The diagram above left is the elevation radiation pattern at an angle of 90 degrees for the inverted-V at 16M (black line) and 6M (blue line). This is essentially the pattern in the axis of the antenna, where the radiation is minimal for a dipole. It can be seen that at a height of 16M, the inverted-V is still directional.

The picture above right shows the antenna configured as two independently fed crossed inverted-V dipoles for 10m.

Antenna Performance

I've been impressed with how well these antennas have functioned considering the low cost and ease of construction (see my Contact Logbook).

The main issue I've come across is problems tuning in the 10m element when configured in a fan under 20/17 and 15m elements. I suspect this was due to interactions between the elements, but was easily solved by offsetting the dipole elements.

Using PVC horizontal support for flat-top 10M dipole

Previous iterations of the dipole antenna I've been using


The picture above right shows some of the various configurations I've run this antenna, and above left the antenna when configured as a single flat-top for 10M. I used PVC central-heating poles to provide the horizontal support with the dipole and balun underneath. However, guy wires are required to stop the PVC poles sagging, but they still did bend lots in the wind!

Buddistick Vertical - HF

Buddistick mounted on ex-Sky dish mount

My second antenna on HF is a Buddistick1) covering 40m-6m. The antenna consists of a short whip, tuneable coil and counterpoise - totalling 250cm in length.

I started out with this antenna before installing the dipole above - it's actually meant to be a portable antenna but is ideal for a location with limited outside space.

It's mounted to an old Sky dish bracket using the universal clamp that comes with the Deluxe Buddistick package, with a single counterpoise strung out to the corner of the roof terrace.

I have the counterpoise guy wire double up as a marker for the HF bands from 40m-10m, so can quickly re-tune the antenna when changing bands. The whole antenna can be quickly dismantled and packed away.

Tuning comprises of moving a detectable clip up and down the coil, and changing the length of the counterpoise. I've included below some settings I use for the different bands.

Coil & Counterpoise Settings

I use the below guide to tune the antenna, then fine tune by winding in/out the counterpoise.

Band Coil Tap 2) Counterpoise (mm)
20m 12 400
17m 8 320
15m 5 318
12m 4 254
10m 1 224

Antenna Review

Before purchasing I spent a few weeks researching antennas as my original requirements were quite specific:

  • Good coverage across the HF bands
  • Easily tuneable with no access to earth/ground for large radial system
  • Small enough to be mounted to a Sky dish bracket (rented property, so can't drill any holes!).
  • Easily collapsible
  • Good performer for the size

The Buddipole and Buddistick came out very well in the reviews (and ticked all the boxes above), and there are a whole host of videos on YouTube showing them close up. I eventually opted for the Buddistick Deluxe, costing just short of £200 (pictured right).

The antenna comes with a small case for easy transport, and I've been very impressed with the quality of all the items and attention to detail (even the instructions come in a waterproof cover!). Assembling the antenna is very easy, and it's not too heavy.

The Deluxe package adds a few extra coil clips, a spare whip, the universal mount and the carry case - well worth the extra £50.

The tune by ear method works a treat, leaving you to finely tune for good SWR by changing the length of the counterpoise.

I'd definitely recommend this antenna, both as a portable and as a small inner-town antenna. htmlhtmlhtml/smallhtml/small htmlhtmldiv style=/smallhtml/smallhtmlhtml/smallhtmlsmallhtmlhtmlhtmlhtmlhtml/divhtmlhtml/smallhtmlbr /html/div

1) The Buddistick Deluxe package adds an extra whip, universal mount, extra coil clips and comes in a handy case
2) As measured from the whip end of the coil
 
m1bwt/antennas.1328908532.txt.gz · Last modified: 2012/02/10 21:15 by simon
 
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