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Ever wonder why some 50 ohm loads are reasonably priced, look great at dc, but quickly veer off 50 ohms at higher frequencies? Take a look at the attached photo, courtesy of one of the Russian NanoVNA boards, and wonder no more. The ground lead even looks it is compression contact instead of soldered.

- Herb

Hello All,

Could someone advise if these SMA terminators are good ?

I've done a resistance check at DC using multimeter and it's shown 68 Ohms (too high by me, should be lower).
Secondly, I've attached to VNA (properly calibrated with other known good 50 Ohm SMA) and the result is:

1. At frequencies lower than 100Mhz SWR is 1.3.
2. In 1Ghz range has significant capacitance impedance.

The item is rated DC-18Ghz TRM-2444-M0-SMA-02 (specs https: www.mouser.co.uk/datasheet/2/643/pi-CCS-MW-TRM-2444-M0-SMA-02-1312942.pdf)

Am I doing something wrong or terminators are defective ?
Pictures attached.

Regards,
Constantin

Hmmm...looks like a Nano-Cantenna ;-)



On Sat, 25 Jan 2020 at 3:34 PM, hwalker<herbwalker2476@...> wrote: Ever wonder why some 50 ohm loads are reasonably priced, look great at dc, but quickly veer off 50 ohms at higher frequencies?? Take a look at the attached photo, courtesy of one of the Russian NanoVNA boards, and wonder no more.? The ground lead even looks it is compression contact instead of soldered.

- Herb

On Sat, Jan 25, 2020 at 01:08 PM, <cosbug@...> wrote:

Could someone advise if these SMA terminators are good ?

I've done a resistance check at DC using multimeter and it's shown 68 Ohms (too high by me, should be lower).
Secondly, I've attached to VNA (properly calibrated with other known good 50 Ohm SMA) and the result is:

1. At frequencies lower than 100Mhz SWR is 1.3.
2. In 1Ghz range has significant capacitance impedance.

The item is rated DC-18Ghz TRM-2444-M0-SMA-02 (specs https: www.mouser.co.uk/datasheet/2/643/pi-CCS-MW-TRM-2444-M0-SMA-02-1312942.pdf)

Am I doing something wrong or terminators are defective ?

===========================================================

Constantin from your attached photos you have a reason to be suspect. The low cost terminator that came with my NanoVNA-H4 has a return loss of over 40 dB up to 1 GHz, and the Midwest Microwave terminator in your photo is showing a return loss of 20 dB at some frequencies. That's pretty poor and definitely outside the specifications of the device.

I have come across similar Midwest Microwave terminators at swap fests and I cherry pick them using the NanoVNA I always carry with me. The sellers aren't very thrilled because they are left with known subpar terminations on their table while I walk away with the in spec devices.

- Herb

68 ohms (resistance) will give you a VSWR of 1.36, so it's very nearly in
spec, according to the soec sheet you linked.

-. Aldo

On Sat, Jan 25, 2020 at 02:31 PM, Aldo Cugnini wrote:

68 ohms (resistance) will give you a VSWR of 1.36, so it's very nearly inspec, according to the soec sheet you linked.
=============================================

Aldo,
If you read the spec's closely its:

VSWR: 1.05+0.008f(GHz) max@...,
1.30max@...

So in the provided 50k - 1 GHz plots, the return loss should be no smaller than 30 dB (~VSWR 1.06).

- Herb

Herb,

Making reasonable quality loads is not difficult.? See if interested.

Larry, W0QE

On Sat, Jan 25, 2020 at 03:10 PM, Larry Benko wrote:

Making reasonable quality loads is not difficult.? See if interested.
==========================================================================

Larry,
I've made a few like that myself and they all perform well up to 1 GHz. My main reason for the post was to show that if you measure a termination with a dc meter and have it read 50.01 ohms that it isn't necessarily good for use at VHF or UHF frequencies. A lot of inexperienced users don't understand that, but seeing a poorly assembled terminator makes the concept easier to remember.

- Herb

On Sun, 26 Jan 2020 at 00:08, hwalker <herbwalker2476@...> wrote:

Larry,
I've made a few like that myself and they all perform well up to 1
GHz. My main reason for the post was to show that if you measure a
termination with a dc meter and have it read 50.01 ohms that it isn't
necessarily good for use at VHF or UHF frequencies.

My company makes VNA calibration kits. I don't measure the DC resistance
at all - I just ensure that the return loss meets its specifications.

I see so many people on here believing that they are measuring the SWR of
their own loads, I give up replying, as I would never have time to do any
real work.

Dave
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom

Thanks Herb for confirming my suspicion.

I've looked at the specs once again: VSWR: 1.05 + 0.008 f (GHz) max @ DC - 18.0 GHz, 1.30 max @ 18 - 26.5 GHz
so I would expect 1.30 at frequencies above 18GHz, which is not my case.

PS: I did compare with Radiall SMA terminators rated up to 4GHz and I don't see such behaviour at low frequencies.

Hello group!
I am a little confused now regarding what a typical DC resistance should be for a 50 ohm load that the specs say DC to whatever. At DC shouldn't it be very close to 50 ohms? A measurement of 68 ohms at DC is 36% high ? What am I missing here ?

Vidas

On 1/27/20 8:02 PM, N8AUM via Groups.Io wrote:

Hello group!
I am a little confused now regarding what a typical DC resistance should be for a 50 ohm load that the specs say DC to whatever. At DC shouldn't it be very close to 50 ohms? A measurement of 68 ohms at DC is 36% high ? What am I missing here ?

???? Yes, it should measure 50 ohms.? Your load appears to be defective.

Constantin,

Clearly something is seriously wrong with your termination.
Because the poor performance extends all the way down to
DC (wrong resistance value), I'd say that the termination in
question has either been severely damaged, such as by
excessive power having been applied, or Mouser did not
shipped what they advertised.

From the specs (and the price), it ought to have been a quite
decent termination. Midwest Microwave is a long-time &
well-respected producer of such items.

I bought a precision N termination directly from Midwest back
when I lived in Ann Arbor (circa 1980) and their facility was
within bicycling range of my home. I made arrangements by
phone, then picked it up in person. I still have (and love) that
termination.

Dana K8YUM

Thanks Dana for confirmation,

I've initiated return of items.
By the way, the visual inspection doesn't unveil any mechanical or heating issues. They look like new.

Regards,
Constantin