VU2PPP
A-655
Sector-19
Noida, U.P 201301
ph: 09811212386
alt: 09811971176
praveen
The SKY LOOP Antenna (SLA)
Hundreds of references are available on how to install / tune full wave SLA. But there is no standard way of doing it!
Everybody has his/her own logic and experience of installation and tuning of SLA. But one thing is common in all the articles – the SLA is one of ‘Best’ performing multi-band antenna available today!
This article presents a strait forward method of installation and tuning of SLA for 40/20/15/10M bands and the results obtained.
Step 1
Preparing λ/2 length cable for 40M
For this you’ll need Antenna Analyzer or RF Impedance Meter or SWR Meter or a GDO.
Cut about 46.5’ of 50Ω cable (like RG58 A/U) to start with. This length is λ/2 X .66 (velocity factor of cable). A number of articles are available on the net on how to make a λ/2 cable; so this is not being covered here.
Step 2
Installation of Sky-Loop Antenna
Install full wavelength loop horizontally above the ground (if real estate is available) or on the roof top; as high as possible. Cover as much area as possible.
Connect the λ/2 cable at the feed point.
Connect the Antenna analyzer at the other end of the cable and check the resonance frequency.
Trim antenna loop length by 4 to 6” each time to bring the resonance frequency at 7.0 Mhz.
Important
At resonant frequency the reactance becomes Zero, input impedance could be any value
The antenna is now ready for use. Simple!!
Why this approach?
The feed point impedance of resonant loop, cut for 40M, is nowhere around 50 ohms for 20, 15 or 10M bands. And this has to be fed with a 50ohms coaxial line! It is not easy to install a multiband antenna tuner at the feed point high up in the air and tune it remotely. So the next best thing is to bring this feed point impedance inside the shack by using a coaxial cable cut to multiples of λ/2.
Now if you are OK with about 1 to 2.5 SWR on all bands; you do not need to do any further trimming or matching – just go on with happy DXing!
If you were not using λ/2 or its multiple length for feeding the sky-loop, the feed point impedances would be anywhere and antenna would perform in an unpredictable manner.
The following graph shows the results obtained for the 40M SLA installed at my QTH on the roof-top of the 2nd floor.
X-axis is Frequency in Mhz, Y-axis is SWR
Did you notice the 2M line !!
Yes, 40M Sky-loop works wonderfully well on 2M also !! Check it out to believe this !!!
dedicated to those who tinker around in their work shop to innovate and create .....
A-655
Sector-19
Noida, U.P 201301
ph: 09811212386
alt: 09811971176
praveen