This article describes a simple end fed wire for the low HF bands installed at a holiday cottage.
Priority requirements were a single antenna system that is unobtrusive and:
To achieve reasonable performance down to 80m, the wire needs to be longer than about 25m.
In this case, a path for a wire of 25 metres generally upwards in an eastwards direction was identified and the antenna rigged. For practical reasons, the wire is vertical for the first 4.8m, and then hangs in a sloping catenary to a height of 17m. Overall, the wire rises 17m in a horizontal run of 14m, so it is predominantly vertical which in the low noise environment of the coastal village of Narooma is quite acceptable.
Table 1 shows the modeled feedpoint impedance on the HF amateur bands, the settings and loss for an L-match tuner, and feedpoint voltage at 100W it can be seen that a 25m wire is barely adequate for reasonable performance on 80m, and definitely lacking in performance on 160m.
The Icom AH-4 tuner manual requires avoidance of half wave lengths of radiator, suggesting that it subjects the tuner to excessive voltage.
Figure 1 is a plot of the modeled feedpoint voltage over the HF range. Whilst the voltage climbs rapidly below 3MHz (which is the quarter wave resonance of the wire), it is well contained at all frequencies above 3MHz, and should not be an issue even at the maxima. I am a loss to understand why the advice in the user manual, especially given that the feedpoint voltage on a 6m vertical at 3.5MHz is around 6KV. Nevertheless, I defer to their advice, and have selected a wire length that avoids operation at the voltage maxima on any of the amateur bands.
The L-match model used a capacitor Q of 2000, and inductor Q of 100 at 1MHz and proportional to the square root of frequency. I note that many estimators of such a network use a constant inductor Q, and others make it proportional to frequency. Since inductive reactance is proportional to frequency, and skin effect causes coil resistance to be proportional to the square root of frequency, and Q is Xl/R, then it seemed to me that the best model for this situation was to make Q proportional to f/f^0.5 or the square root of frequency. This is a more relevant issue at low frequencies and for larger inductances.
In general, tuner loss is not an issue except at the very low frequencies where the load has low resistance and high capacitive reactance, requiring a large tuning inductance with attendant high series resistance operating at high current increasing the inductor losses markedly.
The AH-4 is not rated for use on 1.8MHz and in the event will not tune the load. Indicated loss for an L-match on 1.8MHz is 0.9dB, at which 20% of the transmitter power is dissipated in the tuner inductor and which would be of concern.
On the bands that are within the specification for the AH-4, worst case loss is less than 0.15dB or 3.5%.
I do not expect that T-match tuner will generally perform as well as the L-match figures above, especially for the very popular models with small capacitors (250pf or less) at low frequencies. Initial modeling indicates around 3dB of loss to T-match at 1.8MHz and 0.4dB at 3.6MHz.
A significant issue is that on 1.8MHz with such a T-match, 40% of the transmitter power is dissipated in the tuner inductor which may well damage the inductor. Nevertheless, indications are that a T-match would probably be satisfactory on 3.6MHz and higher bands.
When I get a few spare moments, I will write a program to auto-tune and analyse a T-match and include the results.
Performance is the overall result of:
Figure 3 shows the modeled EIRP over the HF range. The antenna performance is good over the range 40m to 10m, and tolerable on 80m, but 160m is seriously low due mainly to low feedpoint resistance relative to the earth resistance, and the tuner losses in matching such a low resistance.
Directivity is general, the antenna does not generally exhibit strong lobes in any direction, and reporting 3D gain is not very relevant unless specific circuits are being considered. The primary use is for local contacts (0 to 1000Km or so) where circuit requirements are a radiation angle of 25 to 90 degrees.
The antenna was modeled with EZNEC to obtain an indication of the radiation pattern. Table 1 shows modeled gain at 45 degrees radiation angle. The Table 2 is a summary of the patterns on the lower bands.
|1.8||omni directional, distinct low angle major lobe|
|3.6||omni directional, distinct low angle major lobe|
|7.1||fairly omni directional, small hole in the direction of the wire, fairly omni in elevation.|
|10.1||good broadside with mid angle broad major lobe, mainly low angle and high angle lobes end on|
|14.15||dominant lobe at zenith, bit of a cloud warmer, not optimum for long distance low angle circuits|
|18.1||good broadside with mid angle broad major lobe, mainly low angle and high angle lobes end on|
|21.2||complex pattern of lobes, significant lobe at zenith|
The earth system resistance is significant at low frequencies where the ESR of the antenna is very low.
Figure 4 shows the modeled equivalent series resistance (ESR) (including earth system resistance) over the HF range (note the logarithmic scale). The ESR is a little low in the 80m and 30m bands, and is very low in the 160m band
The wire is 2mm diameter copper. Increasing the wire diameter substantially made little difference to the gain in the EZNEC models, so wire loss is discounted as insignificant.
Matching network loss is significant on the very lowest band, and typical losses for a practical L-match are shown in Table 1.
The intention is to match the antenna to the feedline at the base of the antenna. In that circumstance, the transmission line runs matched, and the length of 4m of RG58 has insignificant loss over the whole HF range.
Because of the proximity to the ground, the vertical section and the sloping section, the pattern will not be strongly directional.
Use of a wire in non-resonant mode with an ATU provides frequency agility. The Icom AH-4 provides very convenient frequency agility. A manual L-match tuner is less convenient, but quite effective.
The following illustrates the physical implementation with some accompanying notes.
|The antenna is a length of wire which rises
up the outside wall of the building for 4.8m and then forms
a catenary of length 18.4m onto a tree branch at about 17m height. The
actual shape of the wire is modeled as three
straight segments as seen in the diagram. The sloping section is at a
compass bearing 84 deg T.
The antenna wire is 7x0.67mm (insulated) building wire.
|A short length of nylon rope, counterweight and pulley are used to tension the lower end of the catenary. The antenna can pull about 2m of rope to adjust for movements in the tree, or for increased belly under strong wind.|
The nylon rope has an eye splice on the lower end and a D shackle to a 500g Snapper lead (sinker).
The vertical run of wire from the tensioner is supported on two stand-offs which are ceramic bobbin insulators mounted on 6mm stainless threaded rod screwed into threaded masonry anchors.
The antenna wire is passed through the brick wall using a feed through arrangement using 4mm stainless threaded rod sleeved with flexible 4mm irrigation riser, and a pair of Delrin bushes (see detail below).
The earth conductor is fed through the wall using a 4mm stainless threaded rod.
The green cover is a plastic irrigation valve box covering the earth electrode terminations.
|The main earth electrode is a 12mm diameter copper clad steel rod, drive to a depth of 2m (when it struck rock) and cut off just below ground level. There is also two shallow buried radial about 6m in length that connect into the clamp.|
|The antenna is tuned using an Icom AH-4
automatic antenna tuner. It is located on the inside of the
wall adjacent to the feed-through arrangements seen to the right of the
The tuner hangs on a couple of screws, and is easily detached for other use.
|This is the inside of the feed-through
arrangement for the antenna wire.
The brass ferrule is tapped to screw onto the 4mm threaded rod, and is drilled to accept a standard 4mm banana plug for ease of connection and disconnection. For flexibility, it also acts as a conventional binding post.
|A close up of the delrin bushes used for the antenna feed-through.|
|The earth feed through arrangement is just a 4mm threaded stainless rod through both thicknesses of brick.|
The earth system consists of a driven copper clad electrode, and a pair of shallow buried 3mm HD copper wires. The earth system is used exclusively as an RF ground, it is not intended as a protective earth.
Low frequency earth resistance was measured following installation using the three wire fall of potential method which indicated 9 ohms.
The antenna works very well on 80m, 40m, and 30m. Signal reports both received and sent have been in accord with expectations and compare very favourably with other stations over the same path at the same times.
160m performance was surprising, with some good contacts, but light activity prevented comparative assessment. Modeling indicated performance would be down by 9dB or 1.5 S-points, but nevertheless signals well above S9 were received and sent. The 160m contacts were using an Alinco DX-70TH and EDX-2 tuner which easily tuned the antenna on 160m. (Broadly, the EDX-2 is Alinco's competitor to the AH-4.) Neither the AH-4 or MFJ-949E would match the antenna on 160m.
No assessment has been done on the bands 20m to 10m, although the antenna loads up ok. The QTH is on the side of a steep hill with major obstructions to low angle paths in directions of main interest.
© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.