Guy wire sloper antenna suitable for both 40 and 15m bands. Install January 2012.

Always attempting to get the most from any antenna and tower system I decided that there was no 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 antenna that most suited this application was a 40m band sloper as the guy wire to be used was slightly longer than a 1/4 wave length at 7 MHz and the top fed sloper was the most convenient installation having the least impact on the guy wire primary role; guying the tower.

Sloper antenna for both 40 and 15m bands cut into tower 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 half 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 however with the position of the guy wire in this case being a predetermined 55deg this antenna will have to work with load impedance that results.

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 7 MHz the relatively short 21mtr run of RG213 should not introduce too much loss into the system. For example if a SWR value of 3:1 at 7MHz was achieved the additional coax line loses would be 0.28dB  and at 21MHz  the additional lose would be 0.35dB. See Fig #1 and Graph #1  

Fig #1 Comparative coax line loses for a 21 meter run of RG213 coax for 7 MHz and 21 MHz at various SWR values

Graph #1 Comparative system output power for a 21 meter run of RG213 coax for 7 MHz and 21 MHz at various SWR values for a transmitter input power of 100W

The conclusion therefore is that providing an SWR value of less than 5:1 is achieved then there is negligible losses introduce into the system.  

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 7 MHz 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 much measurement and adjustment the guy wire can be more neatly terminated in 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.  

Measurements

The AIM 4170C antenna analyzer produced a display that indicated the antenna was in the ball park with two SWR dips; one just above 7 MHz and the other at the third harmonic frequency of about 20 MHz just below the 21m band.

The analyser sweep of the entire HF band 3 - 30 MHz shown in graph #2 revelled an unexpected useful dip around 5 - 6 MHz (USA/UK 60m band)  

AIM 4170C antenna analyser explanation;

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

Graph #2 AIM 4170C antenna analyser of HF spectrum from 3 - 30MHz.

Graph #3 AIM 4170C antenna analyser of HF spectrum from 6.5 - 7.5 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 40m band. 

Graph #4 AIM 4170C antenna analyser of HF spectrum from 20 - 22 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 15m band.

Operational Performance

Anecdotally the antenna seems to be performing as predicted, that is signals from the east particularly signals from the east coast of Australia (in relation to Western Australia) generally have marked improvement over the multi-band dipole. There is believed to be a mild null in the multi-band dipole on the 40m band in the easterly direction, therefore all that may be being achieved is over coming the deficiency in the multi-band dipole. 

The below plots for both the 40m and 15m radiation patterns were modelled to give some clue to what was actually happening and obviously not to be taken as gospel. The first model of the 40m pattern was a very basic model with only the radiating element and the mast as far as the sloper attachment point, this revealed to an almost omi-directional pattern however when the full tower structure was included the plot below was produced indicating some directivity. Clearly this suggests that the tower and any attached antennas and hardware play a significant part in the final result of the sloper antenna. Confirming the ARRL Antenna Book 18^th Addition description of the antenna and the crucial part that the tower and hardware contribute to the final sloper performance. 

Figure 2 Radiation pattern of the 40-15m sloper at 7.15MHz

Figure 3 Radiation pattern of the 40-15m sloper at 21.2MHz

Conclusion

Despite the less than ideal galvanized steel wire for the antenna radiator, the higher than I had hoped VSWR the antenna performed as it was hoped. A clear 2 to 3 S point improvement in the easterly direction of gain on the 40m band was evident when compared with the multiband dipole. The ATU has no problem matching the antenna at both 40 and 15m. The sloper antenna also can be matched on the 80m band with comparable performance with the multiband dipole and while it shows no advantage it is noteworthy that it works that well. 

For very little cost there is now an additional antenna available that has demonstrating clear advantage for certain conditions where there was previously only a guy wire.  

While too good and convenient an antenna to be ignored, it must be recognised that no two of these antenna are going to be alike due to the effects of the subtitles of each individual sites and as additional antennas are added and other removed so will the performance of the sloper antenna.

 
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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