Using WSPR to compare antennas – part 1
As you can imagine, having built my QRPp WSPR transmitter and getting everything on the air, I started watching the spots database on WSPRnet with even more interest! I was curious to see how well the different output levels were heard and moreover, to see just how far I could get at these lower power levels.
When I first started using WSPR on 30m I had taken a simple approach to a 30m antenna – load up one of the existing antennas and see how it gets out. The choice of antennas was simple – take the 160m inverted L (with a 65 foot vertical section) and use a remote antenna tuner at its base to deal with the impedance match required.
The antenna worked great at the 5w level from my TS-B2000 and WSPR with spots all over the WSPR world – the bottom of the solar cycle taken into account!
Hams being a competitive crowd, I started looking at the different WSPR stations across the US and the quality (distance & SNR) of their spots compared to mine. Larry WB3ANQ came out hands down ahead of my results – stunningly so!
Larry's <1mw WSPR signal had made it to VK6DI in Western Australia while the best I'd been spotted by VK6DI was at 200mw even though the path from Larry to VK6 is almost 4000 km longer than my path!!!
This warranted some investigation!!!!
Digging into Larry's web site at www.wb3anq.com I found that Larry was using a Force12 Sigma-40XK for his 30m antenna.
The Force12 Sigma-40 is a vertical dipole that looks like an "H" on its side. The antenna uses a combination of end loading (the flat ends of the H) plus a center loading coil.
I decided to compare a simple vertical dipole with end loading mounted 10 feet above ground versus my 160m inverted L – both operating on 30m – using EZNEC. The results were very interesting:
30m Vertical Dipole
As expected the vertical dipole is omni-directional with a peak radiation angle of 21 degrees. I modeled a vertical dipole with 14 foot end elements and adjusted the length to get resonance at 10 MHz – this was a rough model for the Force12 Sigma-40.
N6TTO Inverted L
My inverted L is mounted roughly NE-SW with the horizontal flat section running to the SW – in this diagram that corresponds from left (NE) to right (SW). The pattern is much more symmetrical on 160m than the above plot – not really a surprise because the antenna is many wavelengths long at 30m. The antenna is still mostly omni-directional but with higher gain to the NE. The radiation angle on 160m is about 30 degrees but rises to almost 40 degrees on 30m according to EZNEC.
Could the antenna make THAT much difference?
I thought about this for a while and decided to try another modeling step – using VOACAP and W6ELProp to model the effects of the different antenna radiation angles on projected receive signal levels over the path from the West Coast of the US to the West Coast of Australia when using 200 mw output. This is likely model misuse since the VOACAP model was developed for modeling kilowatts not milliwatts and W6ELProp was designed around 100w typical ham transmitter.
Despite the likely misuse, the comparison against radiation angles on each of these models was the same – some projected paths using the vertical dipole but none on the inverted L. Of course, I'd had a number of spots using the inverted L but the model comparison made me suspect that signal strengths would be better using a vertical dipole.
The next step was obvious – build a 30m vertical dipole and see how it worked!