Guy wire sloper antenna suitable for 10m band. Install September 2012.

Always attempting to get the most from any antenna and tower system I decided that there was no good reason that the tower guy wires could not be used as additional antenna radiators. Although the guy wires are galvanized steel, not the first choice for an antenna due to its higher material resistance, they are however a relatively heavier gauge compared to what had been used for the multiband dipole, hopefully negating the poorer conductivity of the steel. The 10m band sloper antenna is based on the experience of the 40/15m band guy wire sloper that is attach to the same tower.

Sloper antenna for the 10m bands cut into tower's south guy wire.

According to the ARRL Antenna Book 18^th Addition there are two sloper design definitions a sloping λ/2 dipole is known among radio amateurs as a 'full sloper' or just 'sloper.' If only one half of it is used it becomes a 'half sloper.' The performance of the two types of sloping antennas are similar: They exhibit some directivity in the direction of the slope and radiate energy at low angles respective to the horizon. The wave polarization is vertical. The amount of directivity will range from 3 to 6 dB, depending upon the individual installation, and will be observed in the slope direction.

The so called sloper antennas all have one thing in common they are problematic when it comes to matching. There are three components that interact to produce the feed point impedance value and therefore the SWR result. First the length of the radiator element or wire from the tower, secondly the angle that the radiating element has with the tower and finally the tower and other attached hardware it's self.

The final SWR is adjusted by altering the length of the radiating wire and the attachment angle. The tower with its antennas and other hardware offers less of an opportunity for adjustment with exception of an improved earth and counterpoise system.

In this case by using an existing guy wire there is in fact only the radiating element length available for adjustment with the position of the guy wire being dictated by other real world considerations. The examples of the sloper antenna that is presented in the ARRL Antenna Book 18^th Addition and other articles show the radiating wire angle as being 45deg.

Having conceded that the SWR may not be ideal the aim was simple to get to as lower value as was possible given the installation constraints and use an ATU to correct any less than ideal SWR figure in so far as the transceiver was concerned. At 28MHz the 21mtr run of RG213 should not introduce too much loss into the system. For example if a SWR value of 3:1 at 28.40MHz was achieved the additional coax line loses would be 0.326dB. See Fig #1 and Graph #1  

Construction

Construction is very simple; the existing galvanized steel guy wire has two insulators cut into it, one at the top as near to the tower as is possible and the other towards the ground attachment end. The length of wire between the two insulators is about λ/4 at 28.5MHz plus a bit. The insulator cut in at the ground attachment end has wire fed through the insulator with a longish tails of about 1 meter for easy adjustment of the final radiator length. After measurement and adjustment the guy wire can be more neatly terminated to the insulator.  

The coax cable is terminated in a weatherproof aluminium box that should be resilient to the most hostile environments requiring little if any maintenance for the life of the antenna.

Antenna testing and modelling.

All result shown are the Sloper antenna viewed with the AIM 4170C antenna analyser as see at the transceiver end of the coax.

AIM 4170C antenna analyser explanation;

Graph #2 AIM 4170C antenna analyser display of the HF spectrum from 20 - 30MHz as would be seen by the transceiver.

Graph #3 AIM 4170C antenna analyser display of the HF spectrum from 28 - 29.8 MHz indicates that while not suitable for directly connecting to a transceiver, with an inline ATU a usable antenna with minimal line loss is available over the 10m band. 

Graph #4 AIM 4170C antenna analyser display of the HF spectrum from 24.80 - 25.00 MHz indicates that the antenna could be directly connecting to a transceiver, however it is intended that ATU will be used for operations on the 12m band.

Graph #5 AIM 4170C antenna analyser display of the HF spectrum from 21.00 - 21.50 MHz indicates that while not suitable for directly connecting to a transceiver, with an inline ATU a usable antenna with minimal line loss is available over the 15m band. 

Operational Performance

Anecdotally the antenna seems to be performing as predicted, that is signals often have marked improvement over the multi-band dipole in some directions. The multi-band dipole experiences deep nulls in it's radiation pattern particularly on both 12 and 10m bands and as it is an easy process to switch between the two antennas it is simply which antenna offers the best signal.

Figure 2 A three dimensional view of the radiation pattern of the 10m sloper at 28.5MHz. Radiation plot was produced by MMANA-GAL Antenna Analyser software.

Figure 3 Radiation pattern of the 10m sloper at 28.5MHz. The pattern has been orientated over the great circle map centred on Northam, Western Australia. 

Figure 4 Radiation pattern of the 10m sloper at 24.90MHz. The pattern has been orientated over the great circle map centred on Northam, Western Australia. 

References

ARRL Antenna Book 18^th Addition

Coax Cable and Line Loss Calculator http://www.arrg.us/pages/Loss-Calc.htm

Loss in antenna conductor materials

This article explores the potential losses of popular conductor materials.

http://www.vk1od.net/antenna/conductors/loss.htm  

SWR(Standing Wave Ratio) Wikipedia http://en.wikipedia.org/wiki/Standing_wave_ratio

The above radiation plots were produced using MMANA-GAL Antenna Analyser software by JE3HHT, Makoto (Mako) Mori at http://hamsoft.ca/  

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