Using a 75 Ohm source antenna in a 50 Ohm spectrum antenna

[liteyear]

Our SSA3021X finally arrived today. I’d like to be able to do rough first pass EMC assessments and have a variety of current probes, H/B-field probes and antennas to suit. Unfortunately, the best option I found for a sub-GHz antenna has a 75 output while the SSA3021X has a 50 input.

When I skimmed the manual prior to purchase I noted it has an option to select a 75 source. So my vague, hopeful plan was to snip off the antenna’s existing connector, attach an N-type and be done with. Now I read closer I see the option only changes the calculation performed for power, not the impedance of the termination.

What am I likely to suffer if I do simply change the connector, with no effort applied to matching? Having the 50/75 selection option in the SA software seems to suggest there’s a suitable use case of some sort, but I’m not sure if what I have in mind is problematic.

We’re only really going to be interested in repeatable, relative measurements, from about 50kHz to 800kHz. We’ll still defer to a test house for absolute measurements. Will a matching adapter make much difference? Are we best to use a 75 N-type, or will a 50 do, or is it irrelevant given the receptacle on the SA is 50 anyway?

[tggzzz]

If you use a 75ohm source/cable with a 50ohm termination, you will get reflections and the standard frequency-dependent response. See any textbook on transmission line theory.

You can avoid that with a passive 75/50ohm pad, which will have a flat frequency response but the loss in it will (predictably) reduce the sensitivity of your measurements.

You could also use some form of active buffer, but its frequency response will become part of your measured values.

I suspect the pad is the best option in your case. That should be easy to make, given a max frequency of 800kHz.
https://chemandy.com/calculators/matching-t-attenuator-calculator.htm
https://chemandy.com/calculators/matching-pi-attenuator-calculator.htm

[liteyear]

I’ve read some textbooks on the topic, but was still hoping I could lean on wisdom from the real world to help decide whether it matters or not. Am I likely to see big periodic dead spots across the frequency range? Or are they of the order that is small compared to the background noise in this range?

Thanks for the tip on the pad.

[Dulus]

With which connector would you connect 75ohm antenna to a 50ohm load?
Dont break the connector leaves
https://www.eevblog.com/forum/rf-microwave/damaged-n-type/

S.A. impedance setting is for the calculation of volts or amps (dBmV, dBuV, dB*A etc.) which is derived from the measured input power.
You still need to provide a 50ohm input to the S.A., which you figured yourself apparently. Im just writing this for the future readers.

Well the ideal return loss for the 50-75ohm is around -14dB. This would ofcourse change depending connector type and frequency.
This will change slightly with the antenna's input impedance though, it may go towards a better value, or it may go to worse.
How much these reflections will change your measurements, i doubt anyone can say.

I suggest using tggzzz's links to calculate a matching pad for 6dB loss and include the loss at your measurement.
At your frequencies, you can even get away with through hole components i believe, and the calculations would be on point.

[tggzzz]

I’ve read some textbooks on the topic, but was still hoping I could lean on wisdom from the real world to help decide whether it matters or not. Am I likely to see big periodic dead spots across the frequency range? Or are they of the order that is small compared to the background noise in this range?

Time to re-read the textbooks, or ARRL Handbook - and apply the standard equations to your situation.

Alternatively, it would take ~5 mins to do a spice simulation.

[TimFox]

Note that actual 75-ohm N connectors have different dimensions from 50-ohm N connectors (presumably supplied on your SA), and cannot mate directly.  The minimum-loss pad from 75 to 50 ohms has 5.7 dB loss, needs only two resistors, and will avoid reflections on the 75-ohm transmission line from your antenna.  I built a simple box for VHF with an F connector on the 75-ohm side and a 50-ohm N connector on the SA side.
See:  http://home.sandiego.edu/~ekim/e194rfs01/minl_atten/minlosatten.html