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An antenna for 7MHz local contacts

Introduction

This article describes the design of an antenna for "local" contacts on 7MHz, including a simple  and efficient matching system that presents a 50 ohm load to the transceiver. 

The design objectives are:

Design

VK1OD is located in Canberra in the Australian Capital Territory, roughly in the centre of the south east corner of Australia where well over half of Australia's population reside, as illustrated in the map to the right.

A little hard to see on the map, Canberra is located about 110Km inland from the coast within the (small) Australian Capital Territory border that is just visible.

The table shows the state capital cities and the path parameters for 7MHz communications at 1700 local in March / April 2002 with  SSN=100.

Note that radiation angles from 25 to 70 degrees suit these cities, but radiation angles up to 80 degrees are used for contacts at 100Km.

City

Population
 (Million)
Distance Bearing Radiation
 angle
Sydney 4.1 242 51 65
Melbourne 3.5 461 232 47
Brisbane 1.5 942 24 25
Adelaide 1.1 958 269 26
The requirement can be simply met by an antenna with approximately omni-directional characteristics.

Situation in a built up area indicates selection of a horizontally polarised antenna for lowest receive noise.

A dipole mounted low to the ground with its legs sloped downwards from the centre was chosen as a reasonably omni-directional horizontal antenna.

A dipole mounted close to ground and with the legs sloping downward from the centre tends more toward an omni-directional pattern rather than the classic pattern for a dipole in free space. The figure at the right shows the modeled pattern from EZNEC.

RG6 was chosen as a transmission line. The table shows the key parameters for a half-wave of the alternative coaxial transmission lines considered.

Note that the loss of 17.37m of RG6, although greater than for 13.98m of RG213, is almost identical in loss per meter of length.

An alternative transmission line configuration of a half wave of ladder line with a cored balun, but it would exhibit slightly worse losses overall than the RG6 which is easier to install and relatively independent of precipitation.

Cable

Length Matched
Loss
Loss with
VSWR=1.5
RG-58C/U 13.98 0.51dB 0.56dB
RG213 13.98 0.23dB 0.25dB
RG59 13.98 0.40dB 0.43dB
RG6 17.37 0.30dB 0.32dB
Whilst the centre feed impedance of a half wave dipole in free space is just over 70 ohms, installation of the dipole close to lossy earth and sloping the legs down from the centre have the effect of reducing the feed impedance to around 65 ohms which will cause standing waves on a 75 ohm feeder (VSWR=1.15).

The matching strategy is to shorten the dipole to excite a VSWR(75) of 1.5 in the RG6, and cut the cable at the most convenient length where the impedance is transformed to 50+j0. The first point at which the impedance is 50+j0 is at 3 metres from the feed point which doesn't suit my installation. This condition repeats every half wave or 17.5m, so the next point is at 20.5 metres, which does suit. The Smith Chart illustrates the matching scheme.

The model shows  a gain of 5.5dBi at 60 deg elevation on the major lobe. The feed system has quite low overall loss, resulting in 92% of the transmitter power reaching the antenna. The achievement of a 50 load impedance for the transmitter, allows the transmitter to achieve rated output without using a lossy ATU.

These are design model figures, which are the starting point for implementation. Implementation will be influenced by real ground conditions, proximity of other structures etc. It will be necessary to jiggle leg length and transmission line length to achieve low VSWR. Note that there are many types of RG6 and the velocity factor varies with the construction. The actual length of the transmission line will depend on the velocity factor.

An EzNEC model of the antenna / transmission line is available for download.

Frequency 7.080MHz
Height of feedpoint 11.00m
Leg length 10.03m
Leg slope 30 deg
Centre Z 60-j26ohms
Transmission line RG6 (Belden B1189A)
Transmission line VF 0.83
Transmission line Zo 75ohms
Transmission line length 21.1m (216 degrees)
Transmission line loss (mismatched) 0.41dB
Load Z (at tx) 50+j0ohms
Bandwidth 7.000MHz - 7.150MHz for VSWR<1.3
Power handling At 400W pX, the voltage at the voltage maximum is well within specified voltage breakdown limits, and at 120pY, dissipation is about 1W/m at the current maximum. (Stated power levels are the maximum permitted under Australian licencing.)
EIRP for 100W transmitter 325W at 60 deg elevation broadside (best),
220W at 60 deg elevation end-on (worst)
A Smith Chart view of the matching scheme is shown to the right. The values plotted here are as constructed and measured rather than modeled as above, so there is some slight discrepancy. (Click on the picure for a larger, readable, printable image).

The chart is normalised to 75 ohms. The desired normalised impedance at the transmitter end of the line is 0.66+j0. The normalised feedpoint impedance is 0.82-j0.29, and plotting the constant VSWR circle, it intercepts the real axis at Z'=0.66 at a length of 0.94 wavelengths or 34° of line. This length is not practical, so it is extended by 180° to give a line length of 214°. 

Implementation

The centre of the dipole is suspended by a 2mm FSWR halyard through a pulley attached to a masthead gibbet which keeps the gear clear of the mast.

The dipole conductor is 2mm HDC. The legs of the dipole slope down from the centre and at an angle of 30 degrees to the horizontal.

The RG6 transmission line is coiled to form an RF choke to reduce current on the outside of the coax. The coil is two layers of 6 turns each wound on a 90mm former, and adjusted for self resonance at 7MHz.

A short length of 13mm irrigation pipe is used to support the cable and coil, and the folded over end is used for a weatherproofed transition from coax to flexible tails to the dipole elements.

The antenna above the 7MHz dipole is a 2m/70cm vertical fed with LDF4-50A just visible behind the mast.

As a result of damage by birds, the above balun was subsequently replaced with a W2DU style balun as shown here. The balun consists of a dozen Jaycar LF1258 cores threaded over RG6 coax, and the whole feedline (including balun) is fitted inside 13mm PE irrigation tube (which the birds do not attack). It was necessary to remove the jacket for the last 300mm of the RG6 cable to permit the LF1258 sleeves to slide over the shield, and stranded insulated tails were soldered to the coax for connection to the dipole ends. The picture at right is of the system after three years exposure to weather and birds!

The end of the dipole is insulated with glazed ceramic egg insulators and tensioned with a 2mm FSWR to the anchor point. The insulators are sold for electric fences at local rural stores.

Permanent fence strainers (again from the rural store) provide a convenient means of adjustment of the wire tension.

It is important to not over-tension the wire. This type of winch adjuster could develop enough tension to break the wire, but in fact applying more than a 10N static tension risks failure under strong winds.

Comparison with ATU / 50 ohm coax / Balun

The following compares the design RG6 configuration with the very popular ATU / 50 ohm coax / cored Balun solution.

  RG6 ATU + RG58/CU + Balun ATU + RG213/U + Balun
ATU Not used 0.4dB 0.4dB
Transmission line 0.41dB 0.77dB 0.35dB
Balun Included in transmission line
loss above
0.3dB 0.3dB
Total 0.41dB 1.47dB 1.05dB
Cost $10 $468 $520

Other bands

The antenna is designed to be a single band antenna, that is simple and provides excellent performance for contacts up to 1000Km.

It is frequently suggested that the antenna will load up ok on other bands with any reasonable ATU. One needs to be aware that just because an ATU indicates a good VSWR, that is not an adequate indication that all is working well. Three scenarios are analysed below.

3.6MHz

The modelled driving point impedance is 7.5-j770 ohms. It is fed by 21.1m of RG6, which has a loss of 0.25dB at 3.6MHz. There is 16dB loss on the transmission line in the mismatched state, impedance seen at the ATU end of the transmission line is 3.43+j35.52 ohms. A T match should transform this to 50 ohms with a loss of around 3dB. Modelled antenna gain at 3.6MHz is about 3dB lower at 60deg elevation compared to performance at 7.1MHz.

So, all up, it is 22dB or so worse in performance than at 7.1MHz - good enough reason to not use it!

10.1MHz

The modelled driving point impedance is 314+j597 ohms. It is fed by 21.1m of RG6, which has a loss of 0.41dB at 10.1MHz. There is 3.1 dB loss on the transmission line in the mismatched state, impedance seen at the ATU end of the transmission line is 10.80+j46.97 ohms. A T match should transform this to 50 ohms with a loss of around 0.3dB. Modelled antenna gain at 10.1MHz is about the same at 60deg elevation compared to performance at 7.1MHz.

So, all up, it is 3.2 dB or so worse in performance than at 7.1MHz - perhaps tolerable.

21.2MHz

The modelled driving point impedance is 79-j160 ohms. It is fed by 21.1m of RG6, which has a loss of 0.6dB at 21.2MHz. There is 1.7 dB loss on the transmission line in the mismatched state, impedance seen at the ATU end of the transmission line is 35.90+j71.57 ohms. A T match should transform this to 50 ohms with a loss of around 0.1dB. Modelled antenna gain at 22.1MHz is about 2dB better at 60deg elevation compared to performance at 7.1MHz, though more directional.

Although this is harmonically related to the design frequency, the combination of dipole shortening and using the transmission line as an impedance transformer prevent the use of this antenna on 21.2MHz without an ATU.

So, all up, it is similar in performance to 7.1MHz on its major lobe, but more directional.

Reports

Achievement of the best VSWR depends on the time one is prepared to spend trimming the antenna legs and cable length. In my case, I abandoned efforts when the VSWR at 7.1MHZ was 1.1. The VSWR rises to 1.4 at 7.000MHZ and 7.2MHz. Whilst pursuing very low VSWR mid band might seem a waste of time, the benefit is mainly in obtaining the best working bandwidth.

Signal reports indicate that the antenna is working as intended. Receive signal strength and reports received from other stations are consistent with similarly configured other local stations, and the paths commonly encountered.

Though I have worked the odd North American, European, and Japanese station with reasonable strengths, the antenna is optimised for contacts ranging up to 1500Km, and works very well over that range.

Similar schemes

A similar scheme could be applied to a folded dipole and a suitable open wire feeder. For example, to use a tuned length of Wireman 551 which has a Zo of close to 400 ohms to transform the feed point impedance to 50 ohms, a VSWR of 400/50=8 would be required. There are some practical issues about measuring the VSWR on the 400 ohm line.

Summary

The advantages of this design are:

Update

I have moved from the location described in this article. My new location has quite similar propagation requirements, and my new antenna is described at Bowral 7MHz dipole for local contacts. Not all of the content of this article appears in the new article.

Glossary

Term Meaning
ATU Antenna Tuning Unit - in this context, a device for transforming the impedance presented at the transmitter end of the transmission line to a load impedance to suit the transmitter.
Balun Balanced - Unbalanced - a device for transitioning between balanced and unbalanced transmission line, may take many forms.
FSWR Flexible Steel Wire Rope
HDC Hard Drawn Copper
SSN Sun Spot Number
VSWR Voltage Standing Wave Ratio
   

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