F-connector or SMA - which is best?

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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

F-connector or SMA - which is best?

Postby DaveB » Fri Jul 17, 2015 8:48 pm

In my opinion changing to an SMA connector was not a good idea. Yes it puts it in line with other SDRs but what about the cable you are forced to use with it.

RG58 or RG 59 has as a loss quoted around 6 dB per 100 ft at 100 MHz - and is the cable you would use with an SMA connector. BTW there are a couple of nice videos on Youtube showing you exactly how to fit both male and female crimp-on connectors.

WF100 satellite cable - has a typical attenuation of 6db per 100 metres - one third the loss - and is the one you would use an F-connector with.

Myself - if I buy an SDR with an SMA connector I will use and SMA/F connetor adapter. But what about the fact that RG58 is 50 ohm impedance and WF100 is 75 ohm - or that most professional/amateur radio equipment is 50 ohm and consumer electronics including FM is 75 ohms.

The sma connector might be better at higher frequencies but RG58 cable loss is horrendous

What effect will feeding SDR Play with a 75 ohm impedance have. If you try and work out a bandpass filter using the on-line calculators you get entirely different results in terms of the capacitance and inductance you need when you change the characteristic impedance.

It is a veritable mine-field.
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zanco
Posts: 3
Joined: Mon Jul 13, 2015 11:30 am

Re: F-connector or SMA - which is best?

Postby zanco » Sat Jul 18, 2015 1:32 pm

Hi Dave,

Given the fact that most antenna's are 50 Ohm of impedance I would prefer a 50 Ohm connector on my SDR.

I surely agree that RG58 is not done for higher frequencies, at larger lenghts. For me, I bought some 40 cm SMA patch cables which I adapt to my antennas, filters and preamps (which mostly come with a SMA connector these days).

I run Ecoflex 15 and RG 213 to the antenna's, short lengths of about 8 meter, and have the SDR receivers at the top of the house, with 3 m USB cables connected to my computer.

Kind regards,

Ben
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F1BJB
Posts: 108
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: F-connector or SMA - which is best?

Postby F1BJB » Sun Jul 19, 2015 10:58 am

The main problem with F or SO239 connectors is continuity of the shielding.
As SDRplay is using a plastic box this doesnt seems very important.
Mine is wrapped in aluminium foil :P
SMA has for me the advantage that my junk box is full of short professionnaly made cables.
It simplifies connection to filters or attenuators.
For long runs the type of connectors is dictated by the cable type and we have to use adapters anyway.
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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

Re: F-connector or SMA - which is best?

Postby DaveB » Thu Jul 23, 2015 8:03 pm

Mine is wrapped in aluminium foil as well - I was thinking of asking the SRPlay team for a stick on logo :D

It has made a lot of difference for supressing any spikiness. My one is an original blue/box/f-comnnector and the aluminium foil os firmly held in contact with the outer of the F-connector with a cable tie. At the other end the foil is not in contact with the usb connector to avoid a ground loop.

Elsewhere in this forum you will see why they chose a plastic box and not a metal one - which makes good sense.

Rgds

David
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