Testing transmitter/reciever pairs "off the air"

[jgalak]

I am fairly new to both electronics in general and ham radio in particular, and this seems like something that should be common, but I can't figure it out yet.  This may belong in the beginners section, if so, please let me know and I'll move it there.

I'd like to experiment with transmitters and receivers without broadcasting.  So I need a way to feed the output of a transmitter to the input of a receiver.  At this point, I'm looking at fairly low power transmitters (certainly under 5W, mostly under 1W), but I would like to be able to work with VHF (2m) and UHF (70cm) bands, as well as HF (mostly 40-10m).  The transmitters is what I'm experimenting with at the moment, the plan is to use commercially produced receivers to "receive" the signal.  The current plan for VHF/UHF is to use a USB receiver like the RTL-SDR or a FunCubePro+, and to use my Flex 6500 for HF.

Every receiver I've seen says, in large warning letters, to not connect a Tx directly to the antenna input, which makes sense.  So what's the solution?

I had one fellow ham tell me that there are dummy loads available with a tap to which a receiver can be connected, but I've not found one yet (there are plenty with taps for attaching a meter to check output power, but that's obviously not the same).  So what's the solution?

Poking around, it seems that one option is to use a BNC (or similair) T-connector to connect the radio to a dummy load, and then use the an attenuator to connect the receiver to the T.  Or do you just go Tx->attenuator->Rx? 

If using a T, how do you determine how much attenuation is required? 

From what I can find, the RTL-SDR dongle has a max input of 10dBm (0.01W) (based on http://superkuh.com/gnuradio/R820T_datasheet-Non_R-20111130_unlocked.pdf).  If my output power is, say, 100mW, then for the Tx->attenuator->Rx I would need a 10 dB attenuator.  If output power was 1W I'd need 20dB.  Can I do that by using two 10dB attenuators in a row?  And for the setup with a T, I simply don't have any clue on how to calculate this...

I've also seen mention of directional couplers used for this, and found this one via google: https://www.rfparts.com/splitter/zfdc-20-5-23.html which seems to be relvant.  If I understand it correctly, the signal at the tap will be 19.5dB lower than the power output of the Tx.  Is that correct?  If so, this may be overkill for very low power transmitters (100 mW range).

Also, while I don't currently own a spectrum analyzer, that's something I'm planning on getting eventually.  If I understand things correctly, a similair attenuator analysis needs to be done when connecting the SA to a Tx, right?  If so, I might as well buy components that are sufficiently high-quality to be useful with an SA later.

Help much appreciated.  Sources for components (US) also much appreciated.

[w2aew]

For testing transmitters without putting a signal on the air, a dummy load is basically essential.  Make sure it is rated for the frequency and power of your transmitters. 

Often times there is enough signal leakage from the coax and dummy load that a nearby receiver will pick it up sufficiently for quick test purposes.  A receiver generally has a lot of sensitivity, so even a greatly attenuated signal will be received well.  As you've been advised, you should never directly connect the transmitter to the receiver.

An RF tap or directional coupler are better solutions than a Tee + attenuators.  You can use attenuators with an RF tap or sampler, or directional coupler, if desired.  And yes, attenuators can be used in series (i.e. two 10dB attenuators in series will be 20dB).  There are some dummy loads that have a tap built in.

I did a video a while ago that talked about different types of RF taps, and also did another video on directional couplers.  I've included the videos below:

[CJay]

Alan's my go to when it comes to things like this and it's well worth spending some time watching his videos (yeah, fan, shoot me!) but I'd add that there are handy plans for building dummy loads and sampling taps on the 'net:

http://www.k4eaa.com/dummy.html

I built one very similar some time ago and it's been most useful but if I were building another today I'd pick up one of the ex cellphone mast dummy load resistors and build it round that, an example, there are many:

https://www.ebay.co.uk/itm/50ohms-DC-3GHz-RF-Termination-Microwave-Resistor-Dummy-Load-RFP-250N50-250W/162423029178?_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D20151005190705%26meid%3D7f0c345d845b44f5b2549018659352dd%26pid%3D100506%26rk%3D1%26rkt%3D1%26&_trksid=p2045573.c100506.m3226

I also built a 40dB tap which I use to get signal into test gear when leakage from the coax isn't good enough (spectrum analyser, frequency counters and my modulation meter which is a devil for picking up odd transmissions but works perfectly with the tap):

http://ae6pm.com/SCCARA-GRAM_Articles/A_40_dB_Power_Tap.pdf

Obviously make absolutely sure you're not going to exceed the input limits on any piece of test gear and add attenuation as necessary, also fit a DC block, especially if you're going to buy/feed a specan that's not rated for DC input (many aren't)

[jgalak]

if I were building another today I'd pick up one of the ex cellphone mast dummy load resistors and build it round that, an example, there are many:

https://www.ebay.co.uk/itm/50ohms-DC-3GHz-RF-Termination-Microwave-Resistor-Dummy-Load-RFP-250N50-250W/162423029178?_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D20151005190705%26meid%3D7f0c345d845b44f5b2549018659352dd%26pid%3D100506%26rk%3D1%26rkt%3D1%26&_trksid=p2045573.c100506.m3226

Huh.  That looks pretty amazing - way more convenient that soldering a bunch of parallel resistors, and no need to worry about capacitive loads between them at high frequencies.  For QRP operations, the heatsink shouldn't even be that bad...

[CJay]

Huh.  That looks pretty amazing - way more convenient that soldering a bunch of parallel resistors, and no need to worry about capacitive loads between them at high frequencies.  For QRP operations, the heatsink shouldn't even be that bad...

They're dirt cheap pulls, you can find them in all sorts of ratings, 250W is a big one.

I've got a 50W one mounted on a P4 CPU heatsink with fan, I've just not got round to finishing it with a connector yet.

Be careful though, some of them are Beryllium Oxide which is deeply nasty if it gets into your body.

[jgalak]

Related question:  one of the radios I want to go into is an RTL-SDR dongle.  That has a 75 Ohm input.  At the powers we are talking about here (sub 5W, probably sub 1W), and at frequencies of 420MHz and lower, does it matter?  Should I bother with a 75 to 50 Ohm adapter (like this one: https://www.minicircuits.com/WebStore/dashboard.html?model=BMP-5075R%2B)? 

[jgalak]

For testing transmitters without putting a signal on the air, a dummy load is basically essential.  Make sure it is rated for the frequency and power of your transmitters. 

Often times there is enough signal leakage from the coax and dummy load that a nearby receiver will pick it up sufficiently for quick test purposes.  A receiver generally has a lot of sensitivity, so even a greatly attenuated signal will be received well.  As you've been advised, you should never directly connect the transmitter to the receiver.

An RF tap or directional coupler are better solutions than a Tee + attenuators.  You can use attenuators with an RF tap or sampler, or directional coupler, if desired.  And yes, attenuators can be used in series (i.e. two 10dB attenuators in series will be 20dB).  There are some dummy loads that have a tap built in.

I did a video a while ago that talked about different types of RF taps, and also did another video on directional couplers.  I've included the videos below:

The RF Taps in the video are mostly variable.  Doesn't that create a problem for measurements, since you can't tell for sure what the attenuation is?

[PA0PBZ]

The RF Taps in the video are mostly variable.  Doesn't that create a problem for measurements, since you can't tell for sure what the attenuation is?

What exactly do you want to do? In your first post in this thread you mentioned 'connecting a transmitter to a receiver' but why do you want to do that? Do you want to measure the transmitter output, and if so, how precise? Or do you just want to listen to the modulation, or something else? While all the advice in this thread is good we can give you an even better advice when we know what you are trying to do.

[w2aew]

The RF Taps in the video are mostly variable.  Doesn't that create a problem for measurements, since you can't tell for sure what the attenuation is?

What exactly do you want to do? In your first post in this thread you mentioned 'connecting a transmitter to a receiver' but why do you want to do that? Do you want to measure the transmitter output, and if so, how precise? Or do you just want to listen to the modulation, or something else? While all the advice in this thread is good we can give you an even better advice when we know what you are trying to do.

If you want to measure sensitivity, or calibrate an S-meter, etc., then you'll need some sort of calibrated tap, or a way of measuring the tap's output.

[w2aew]

Alan's my go to when it comes to things like this and it's well worth spending some time watching his videos (yeah, fan, shoot me!)
<snip>

Yikes, is it bad form to admit you're a fan of my channel?

[jgalak]

The RF Taps in the video are mostly variable.  Doesn't that create a problem for measurements, since you can't tell for sure what the attenuation is?

What exactly do you want to do? In your first post in this thread you mentioned 'connecting a transmitter to a receiver' but why do you want to do that? Do you want to measure the transmitter output, and if so, how precise? Or do you just want to listen to the modulation, or something else? While all the advice in this thread is good we can give you an even better advice when we know what you are trying to do.

Mostly, at this point, test transmitter design.  The immediate project is designing a HAB tracker using APRS.  Possibly other things in the future.  So my concern is to be sure I can do the whole loop - from data generated by a microcontroller, through radio, to decoder software.  For this, a variable tap is likely just fine - I can fiddle with it to get the signal I need.

But long term, who knows?  I may well want to do measurements, and as these parts are somewhat expensive ($50-150 is what I'm seeing), it seems that it's more useful to get ones I can use beyond the immediate project.