RF antenna and band-pass filter matching question - Page 1
Forum section: electronics / projects

author: ealex
author_rank: Frequent Contributor
timestamp: July 06, 2017, 01:31:37 pm
content: HelloAssuming the following set-up: [random antenna] -  -  where:- antenna - random wire / loop / etc- BPF - double tuned filter calculated to work into 50\$\Omega\$- SDR : similar tohttp://qrp-labs.com/images/receiver/receiver.pdfpage 2 - switching mixer.- frequency range under 40MHzI've tuned the filter in a 50\$\Omega\$ system ( improvised with a generator and a 50\$\Omega\$ AD8307 detector) for the desired pass bandwidth around 7.1MHz. Everything relatively matched the theory.After that, I've inserted the filter and performed the following measurements:- [signal generator] -  -  => the filter shape was off => i assume the SDR is not too close to 50\$\Omega\$- [random antenna] -  -  => the filter was way off, i had to re-tune it to the limit of the adjustable coils and wider than it was in the test setupI assume those are effects of the system not following the 50\$\Omega\$ assumption.What should I do to fix this ?Inserting 3db attenuators in front and after the filter should be enough ?If so - should i add some gain after the BPF to increase the signal level after the 2 attenuators ?( I've been using "experimental methods in rf design" as a reference book )

author: PauloConstantino
author_rank: Regular Contributor
timestamp: July 06, 2017, 06:13:50 pm
content: Quote from: ealex on July 06, 2017, 01:31:37 pmjust put some 3db attenuators but match the TL impedance first

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2017, 07:03:44 pm
content: 1. The antenna needs to be matched into 50 ohms, for the filter to work. Random wire antenna, well, the name says it all...2. Additionally, the antenna needs to be matched into 50 ohms over a much wider bandwidth than the filter's transition band(s). A filter works by reflecting undesired energy. That is, energy received by the antenna gets reflected and re-radiated*.3. If the antenna doesn't provide this matching, then the filter must be designed as a diplexer, with the stop-band ports terminated. That is, the energy reflected from the filter is terminated, because the antenna can't provide that termination. Also known as a constant resistance filter (in which case the extra ports might not be designed as ports as such, but are just termination / damping / equalizing resistors thrown in wherever needed).*The antenna re-radiates in the stop band, phase shifted in the process. The reflected energy affects the radiation field around the antenna, so you'll get interference fringes (multipath) in the space around it. You can't hide an antenna just by disconnecting it; after all, it's a hunk of metal, it's going to affect the field somehow.4. Adding attenuators will help -- it gives a sink for the reflected energy -- but it costs signal energy exactly as much. Can you afford the loss in SNR? If you're looking at strong signals, sure; if you want to do DXing, you'll want something better.5. This applies to the SDR as well, but hopefully it varies less with frequency. Can you identify if does vary, or it's just an impedance mismatch -- that's solvable with a mere transformer?Tim

author: C
author_rank: Super Contributor
timestamp: July 06, 2017, 07:17:25 pm
content: One way to match an antenna to your filter is with aTransmatch or Antenna Tunerhttps://en.wikipedia.org/wiki/Antenna_tuner

author: ealex
author_rank: Frequent Contributor
timestamp: July 07, 2017, 07:14:14 am
content: thanks for your answers.i'll measure the SDR input this weekend using this thing:http://qsl.net/n9zia/rlb/rlb.pngorhttp://qsl.net/n9zia/rlb/index.htmlan antenna tuner is not an option right now - the antenna is just some wire loops - i can't install a proper / stable one ( RX only for now )i'll start reading about diplexers - i'll try to build and test one

author: ealex
author_rank: Frequent Contributor
timestamp: July 08, 2017, 09:51:20 am
content: ran a quick test today : looks like the receiver is very close to 50so i need to look only on the antenna side.

author: ConKbot
author_rank: Super Contributor
timestamp: July 09, 2017, 06:42:42 am
content: Yep,a filter with a non-50ohm impedance presented to it will misbehave, and it could be by quite a bit. I've seen a GPS bandpass filter shift so far it turned into a 30-40 dB band stop filter. The attenuation was actually quite impressive, if it wasn't for the fact it was the exact opposite of what I wanted.