Monday, March 25, 2019

Lower noise with HF receiving loops

As important as you being heard by other stations is you hearing them. 

That's particularly the case in urban areas with ever increasing amounts of electrical noise.

160 metre DXers have long used separate receiving antennas to hear weak signals. However the benefits of this also apply on the lower HF bands.

While a shoddily built magnetic loop antenna might not be good for much apart from WSPR transmitting, it can perform well as a receive antenna. This is because signal to noise ratio, rather than efficiency, is most important in a normal environment with ambient RF noise. Loops offer the benefit of having deep nulls that can be used to null out noise if most of it is coming from a single source.

Loops can either be narrow band (like a transmitting magnetic loop) or wide band. Narrow band is good if  you want the loop to act as a preselector. This could be useful on the lower HF bands if you have strong AM broadcast stations nearby and your receiver's front end isn't very strong. However narrow band loops require some sort of variable capacitor which needs to be changed if there is a significant excursion in frequency.  Wide band loops don't have that constraint but don't offer selectivity. 

Another factor to consider is RF preamplification. Very small loops require this for good signal to noise ratio whereas larger loops don't as external pick-up is sufficient. Often the preamp circuitry is near the loop with DC power being fed up the feedline to it. This can be done using a technique similar in TV masthead amplifiers, that is to use an RF choke and capacitor to separate DC from RF. 

Here are some ideas on receiving loops: 

If you try any of these, I'd be curious to hear how you go. Please mention your experiences in the comments below.

PS: Into low power amateur radio? Minimum QRP is the top-selling manual on the equipment, antennas, operating and strategy of successful QRP operating. It's available for under $US 5 each in electronic form. Or you can get a paperback version. Visit VK3YE Radio Books to find out more. 

No comments:

Post a Comment

Upgraded website

Enjoy reading about diverse facets of amateur radio? Like building projects? Sometimes find my videos hard to find? If any of these applies ...