Ken Hough's Website
Amateur Radio

Antennas -- HF Dipoles

This section includes reports on construction and use of dipoles for:
20m band , combined 40m/20m bands, and combined 15m/10m/6m bands

Half wave dipoles are the perhaps the most used, or at least the most aspired to, type of antenna for the HF bands.

Many folks, like me, have only limited space in which to erect antennas, so full length half wave dipoles for the lower bands (160m, 80m, and possibly even 40m) are out of the question. The best space available to me was in my loft.

This page is mainly concerned with my choice of variants of half wave dipole antennas for erection and use within my loft space. I have described the practical problems that I had and how I set up and tuned these antennas to provide me with the means to operate on the 40m, 20m, 15m, 10m, and 6m bands.

As with all antennas, dipole arrangements will not perform optimally unless properly tuned. This is particularly important in non-ideal situations such as in loft spaces where roofing materials, pipework, mains wiring, etc are present -- refer to section below about Tuning Dipole Antennas.

Dipoles are balanced antennas and ideally should be fed via balanced feeder systems. If coaxial (ie unbalanced) feeders are to be used then some form of balun should be included.

A dipole for 20m:
I have just enough space under my roof to take a full length half wave dipole for the 20m band, so this was an obvious first choice -- see A Dipole For 20m below.

What about 40m?
There isn't sufficient space under my roof to conveniently take a full dipole for 40m, so a compromise was necessary. The following possibilities were considered:

1. Another complete dipole which would be erected around (ie looped around) the space available.
I rejected this because of the difficulty of crawling around some parts of the loft space.

2. A 40m "trap" dipole built onto the existing 20m dipole. Although this could have given me a 40m antenna that was somewhat shorter than a full 40m dipole, the outer sections would still need to be looped around the available space, so no real advantage over 1. above

3. A "loaded" dipole built onto the existing 20m dipole. This could be done by fitting inductors (not tuned traps) to the ends of the 20m dipole and then adding short extension pieces (< 1m). This was what I decided to do
-- see A Loaded Dipole For 40m and 20m below.

What about 80m and 160m?
I don't have enough space to erect any reasonably sensible dipoles for operation on these bands, so I am experimenting with a different type of antenna -- see Mag Loops on the main antenna page.

And for the higher HF bands:
Dipole antennas for the higher HF bands (ie 15m, 10m, and 6m) could all be accommodated within my loft space. I decided to experiment with a triple band fan type dipole -- see A Fan Dipole for 15m, 10m, and 6m below.

A Dipole For 20m:

Multi-stranded bare copper wire was used to make up this dipole. To keep the wire reasonably high up and close to the roof timbers, but without any possibility of contact with the timbers, insulated supports were used as shown on the left below. Bases of these insulators were cut from waste pieces of 20mm MDF, and were drilled to take 100mm stubs of PVC pipe. The stubs were drilled to carry the antenna wire.

The outer ends of the dipole were held in position using heavy non-conducting plastic cord.

A centre connecting block was made up from 3mm thick PVC sheet as shown on the right.


The antenna was initially made to be slightly longer than necessary. Then a short loop of wire was connected across the inner ends of the dipole to allow a 'dip meter' to be used to determine the resonant frequency of the antenna. The outer ends of the dipole were gradually trimmed back (actually folded back in case of error) until the resonant frequency increased to nearly the value required. An accurately calibrated transmitter with an SWR meter were then connected in place of the wire loop and dip meter. Trimming back was continued until resonance occurred at the required frequency.

After tuning the antenna, a permanent coax feeder (RG58/U) was installed which included a choke balun at the connection to the antenna.

NOTE:
It was NECCESSARY to tune this antenna, because closeness of the antenna to roofing materials had a significant and unquantifiable effect on resonant frequency. However, after tuning was completed, this antenna showed an SWR of close to 1:1 at resonance.

A Loaded Dipole For 40m and 20m:

As mentioned above, because of limited available space, I decided to build a loaded dipole. The design was taken from the RSGB Communications Handbook, 11th edition, page 15.7. Design details are as follows:

I built this antennas as an extension to the 20m dipole described above. Because of the closeness of roofing materials, initial results were very disappointing. After connecting the loading coils, I had to re-tune the 20m sections, and then tune the outer stubs for 40m operation. My final dimensions were significantly different from those given in the original design, but SWR readings at the two resonant frequencies were close to 1:1.

As might be expected for a "shortened" antenna, bandwidth at 40m is somewhat limited, but given the limited space available, this antenna provides good performance on both 40m and 20m bands.

Instead of the "1:1 current balun" indicated in the diagram, I used a choke balun made up from a few turns of the the RG58 coax feeder cable.

Note:
This is a loaded dipole, not a trapped dipole. It uses untuned inductors rather than parallel tuned traps.
One of the loading inductors is shown connected on the left side to the end of one half of the 20m dipole, and on the right side to the short tail section needed to resonate at 40m. Because of limited space, the outer end of the tail had to be folded by 90 degrees, and is shown held in position by yellow plastic cord.

All joints between antenna wire and inductors were soldered.

This arrangement doesn't look very pretty, but it does work well.


A Fan Dipole For 15m, 10m and 6m:

Fantail dipoles are essentially two or more half wave dipoles for different frequencies that are connected in parallel at the centre feed points, and with the elements "fanned" out to reduce inter-element capacitances.

Provided that the operating frequencies are NOT harmonically related, power will be drawn only by the dipole whose resonant frequency matches that of the RF supply. For example, 15m, 10m, and 6m frequencies are not harmonically related and so can be used together. On the other hand 80m, 40m, and 20m would NOT work together.

The centre block and one set of supporting insulators is shown here.

It is ESSENTIAL to properly tune this kind of dipole arrangement. Initial tuning was done by folding over the ends of the elements rather than cutting to size. This allowed for tuning to be repeated to attain optimum settings on all bands. In fact, only one repeat was needed.

The procedure was not difficult and began with the lowest frequency of operation.

Tuning Dipole Antennas:

For best performance, the resonant frequency of a dipole must fall within the intended operating band. In an ideal free space situation, it should be possible to calculate the dimensions for a dipole reasonably accurately. Many published designs do give specific dimensions. In real situations dipoles are significantly affected by:

-- distance from the ground
-- closeness to other surroundings such as metal pipes and wiring
-- diameter of wire used to make up the dipole

In most cases it will be necessary to adjust the length of a dipole (ie to tune it) to cause it to resonate at the desired frequency. The normal procedure is to begin with a dipole that has been fully erected/installed and that is slightly too long, and then to gradually trim or fold it back (from both ends) until correct resonance is obtained. The problem is how best to determine resonant frequency.

While tuning, long lengths of coax feeder between dipole and test equipment should be avoided as this can cause spurious resonances -- probably not possible for outdoor/high level dipoles. If the centre feed point of a dipole can be easily reached, as in the case of loft a mounted dipole, resonant frequency can be determined by the use of a calibrated "dip meter". A dip meter can be directly coupled to the dipole without the need for a feeder cable. Connect a short loop of wire across the dipole feed points and hold the tuning coil of a dip meter close to the loop, then tune the dip meter to discover resonant frequency of the dipole.

Using Coax Cables To Feed Dipoles/About Baluns:

Dipoles are balanced antennas which are best fed via balanced feeder cables. If a coaxial cable is to be used, then some form of balun should be included close to the antenna so as to prevent RF energy from flowing down the outside of the cable and back down into the shack.

Various forms of balun are possible, but probably the most used types today are:
-- 1:1 balanced to unbalanced toroidal transformers
-- choke baluns

Choke baluns can be made in a number of forms. eg:
-- 15 or more small toroid cores slipped over the outside of a coax feeder and held close to the central connection
    point of a dipole
-- several turns made in the coax feeder cable, again near to the central connection point of a dipole

Most text books that describe antenna design and usage give details for the construction of common types of balun. eg ARRL Handbook and RSGB Communications Handbook

ALL of my dipole antennas are fed using coaxial cables, and ALL are fitted with some form of choke balun. I don't have any problems with RF around the shack.

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13/03/15