Hi Raoul,
Congratulations, its difficult to do better than a 40 dB null. It usually takes a great deal of care with proper loop balance and excellent feed line common isolation to achieve that excellent null depth. 73 Frank W3LPL ----- Original Message ----- From: "Raoul Coetzee" <[email protected]> To: "Richard (Rick) Karlquist" <[email protected]>, [email protected], "Top Band Contesting" <[email protected]> Sent: Saturday, April 2, 2016 9:57:01 AM Subject: Re: Topband: Receive loop observations Rick wrote: > With a >circumference of 20 to 40 feet on 160 meters, the null is only >10 or 15 dB deep. I use a medium wave BC station about 10miles away from as a reference to check my 160m loop, (the loop is reasably broadband but no good on 80m) they are about 4 Degrees East off me and with a nice signal of about 40 db and I easily get a a null signal of S 9. I have played around a lot with my FT1000 MP MKV's S-meter using a step , attenuator, and the meter is most accurate in the range above S9 so I am reasonably happy with my finding of a null of about 40db. Come to think of it, I have an old HP Spectrum analyser that I could use to get a decent measurement. Regards, Raoul Zs1C From: Richard (Rick) Karlquist <[email protected]> To: [email protected]; Top Band Contesting <[email protected]> Sent: Friday, April 1, 2016 11:08 PM Subject: Re: Topband: Receive loop observations On 4/1/2016 1:25 PM, [email protected] wrote: > Hi Jim, > > > A properly constructed loop antenna absolutely requires a preamp. Yes, but usually the preamp function built into the radio is sufficient. > If your loop is operating correctly it will be omni-directional for > skywave signals and it will have an extremely deep null for an > unwanted signal propagated to your antenna from one local > vertically polarized interference source. A well constructed loop > should have a null depth of 60-80 dB and a null beamwidth of just > a few degrees. A very rigid mechanical mount is required to > keep the deep null pointed directly at your interference source. It is easy to model a loop on NEC, and the results I have seen do not predict anything like 60 to 80 dB nulling. With a circumference of 20 to 40 feet on 160 meters, the null is only 10 or 15 dB deep. > A properly constructed loop should be transformer matched > to keep the loop balanced and both the coaxial cable and power > cable must be exceptionally well isolated from the loop. Exactly right > A low noise figure high gain preamp is essential. I've tried that on my loops, and all it does is make the S-meter move more. No audible difference. > Frank > W3LPL > 73 Rick N6RK _________________ Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
