Tagged: TX

Does the RTL-SDR.com FM Bandstop Filter Withstand TX Power?

Thanks to PY2RAF for writing in and sharing some tests that he did on our RTL-SDR Blog BCFM bandstop filter. The RTL-SDR Blog filters were designed for RX purposes only, and no provisions were made for TX with only small SMD components being used. However PY2RAF wanted to test to see if the filter could at least handle 5W. The gist of his results is that the filter seems to handle 5W just fine, but as a precaution we wouldn’t recommend that anyone do this unless you really know what you are doing! 

As he does not have a blog, we present PY2RAF’s write up here:


I am a Ham Radio Operator (PY2RAF), live in a metropolitan and very RF-polluted area.

Recently, I bought a handheld device and was back to the ether, after a 12-year hiatus. I assembled myself a 3-band quarter-wave “cat whisker” antenna for 144, 220 and 430 MHz (https://rf01.co/q/antena.jpg), calibrated it using a VNA and was quickly back up in the air.

Despite great and complimentary reports of good audio and transmission reports, my reception was sub-par: Lots of interference (QRM), static, squelch closing despite high S-bar signal.

I got intrigued by that, it just did not make sense: Had the VX-8 large mouth but bad ears? After a couple of days puzzled, I got a good idea: Put my RTL-SDR.com filter in the antenna.

The result was great: It immediately reduced the idle band noise from 6-7 S-bars to 3-4 S-bars. The squelched interrupted audio also stopped happening.

So, I could conclude that the strong FM BC band was overloading the receiving stage of the radio. Culprit found.

However, it brought another problem: the filter is NOT designed to cope with TX power (it is actually expressly stated at the product description page). However, the enhancement was just too good and I reached Carl asking about TX support or tests. Carl explained me that while the filter was not designed with TX power in mind, it withstand some minor current, because it supports Bias Tee currents.

I took it as a ‘good enough, I’ll test it’. See the results below.


The Device Under Test (DUT) is a RTL-SDR.com FM Bandstop Filter. The transceiver is a Yaesu VX-8DR. I used a PocketVNA Vector Network Analyzer for checking the filter S21 characteristics and antenna S11 VSWR and impedance figures.  The realtime VSWR and TX power were monitored by a Diamond SX-200. I also used a Rtl-SDR.com SDR dongle and GQRX software to check for any transmission distortion. The radiant system (antenna) is a homebrew 3-band multiple dipole antenna, with VSWR < 1:1.3 in frequencies under test.


Prior to any transmission, I put the DUT in the VNA and noted its frequency and attenuation figures.

Next step, assembled the test environment:

Transceiver – wattmeter – DUT – antenna.

I did then the first test: set the radio to its lowest power (0.05 W) and transmitted in frequency 144.320 MHz. I have also tuned the SDR dongle in the same frequency and watched the  waterfall pattern, while listening for the resulting audio. Then, repeated the very same test now adding the DUT before the antenna. The waterfall signature and the audio quality was pretty much the same and coherent. Transmitted for approx. 30 seconds using the Filter.

In the next step, I repeated the tests raising the TX power to 1W and 2.5W. I requested feedback from a fellow Ham operator and got report that the audio quality was pretty much the same with and without the filter, with no changes in RX S-units figures. It means, it did not distort the audio nor put significant attenuation into the signal.

The next test was the real world conditions test. I switched to the repeater 146.910 MHz, negative shift (actual TX 146.310, https://goo.gl/maps/45cUY58yot52). This repeater is located circa 100 KM north from my residence. After introducing myself to the repeater and stating the device test, I started transmitting first with a single watt: successfully hit the repeater. After around 7 comms averaging around 2 minutes, I asked for feedback with and without the filter: The reports that I have heard were of no change in the quality or fidelity of the transmission. The SWR was being continuously monitored by the Diamond SX-200, paying attention for any component disruption and sudden SWR raise: The operation was just normal. The filter also did not present any temperature change noticeable by touch.

Finally, I raised the TX power to 5W and requested report. I did a 1’30” TX and got report of normal transmission.


This test validated, to me, the useful and robustness of the bandstop filter in my antenna as a permanent solution: It did not change the SWR figure, produced heating, noticeable attenuation or signal distortion: It became, since then, a permanent item between my radio and the antenna.

After the tests, I ran another round of DUT tests in VNA and the attenuation of the filter were the same as original: Working the way it should be.

Next day, I joined the repeater net again and spent around two hours ragchewing in the radio, accumulating something around 25 minutes of TX. Nothing wrong was noticed.

A Final Note

It is important to register that the DUT is working in a nicely matched (VSWR < 1:1.5) antenna system. Unmatched or higher VSWR figures can result in higher voltage, enough to break down the isolation. High-Q antenna systems might also present the same issue.

Building an SDR Transmitter using GPIO Pins on an FPGA

Recently an RTL-SDR.com reader named Jon wrote in and wanted to share his project called FPGA-TX. FPGA-TX is software that provides low-cost SDR transmit capabilities on an FPGA. It works in a similar way to RPiTX which is by simply turning the GPIO pins on and off very quickly in such as way that it generates any desired AM/FM/SSB transmission. These methods are crude and require external analog filtering, but can be used for creating almost any sort of RF transmission at a wide range of frequencies extremely cheaply. These sorts of cheap transmitters are great companions to low cost SDR dongles like the RTL-SDR.

Jon’s project runs on FPGA boards and currently supports the Digilent Nexys 4 and Digilent CMOD A7 ($75) FPGA boards. An FPGA is an integrated circuit that can be easily reconfigured to implement various different digital circuits.

FPGA-TX can transmit at frequencies of up to 400 MHz and current supports AM, FM, LSB, USB, Wideband FM and Wideband FM Stereo transmission modes. It runs on Linux. The FPGA transmitter has been tested combined together with an amplifier and filter. It can also interface with a GPS unit for clock calibration.

An FPGA Based Transmitter. In the photo: FPGA, Amplifier, Filter, Attenuator, TX/RX Switch.
An FPGA Based Transmitter. In the photo: FPGA, Amplifier, Filter, Attenuator, TX/RX Switch.
The FPGA-TX Ubuntu Interface.
The FPGA-TX Ubuntu Interface.

Using the RTL-SDR as a Transmitter

Back in July of last year we posted about a video from oh2ftg where he showed how he was able to get his RTL-SDR to act as a crude transmitter by using the RTL-SDR’s leaky oscillator.

Now another RTL-SDR experimenter, Oscar Steila (IK1XPV) has had a similar idea to use the RTL-SDR as a transmitter, and has taken the idea further than OH2FTG did. 

Oscar decided to take a standard RTL-SDR dongle and modify it so that it outputs a signal from the mixer output of the R820T tuner chip. To do this he removes some unneeded components from the PCB, and wires pin 5 of the R820T to the MCX antenna port through a 100pF capacitor. Pin 5 is connected to the mixer output from inside the R820T chip.

TX mod for the RTL-SDR.
TX mod for the RTL-SDR.

After performing the hack the RTL-SDR is able to output a signal anywhere between 500 MHz to 1500 MHz 1.8 GHz to 3 GHz (see why). To control the output frequency you simply need to tune to the frequency you want to transmit at in SDR# (after setting an offset to account for the R820T’s IF offset). This tunes the mixer in the R820T and causes the output frequency to change.

In the future Oscar hopes to take this idea further by creating a specific tuning application for the generator and finding a way to possibly FM modulate the output.

Using SDR# to tune the TX RTL-SDR, and using another instance of SDR# and RTL-SDR to receive the 1GHz signal.
Using SDR# to tune the TX RTL-SDR to 1 GHz, and using another instance of SDR# and another RTL-SDR to receive the transmitted 1 GHz signal.

Update: Oscar has revised the frequency range from 500 – 1500 MHz to 1.8 GHz – 3 GHz. More information about his new tests can be found at http://www.steila.com/SDR/RFgenmod/index.html.

“Worlds Smallest HF Receiver” (KN0CK Integrated RTL-SDR + Upconverter)

Update: KN0CKs products are now available at http://www.kn0ck.com/HF_SDR/.

Yesterday Marty (KN0CK), developer of the miniature integrated HF upconverter for the RTL-SDR that was recently posted wrote into us with some updates.

Worlds Smallest HF Receiver

The version we posted about recently was his “Production 3” version. He writes to tell us that along with his “Production 3” release he created a 15mm x 15mm “Small Form Factor” radio enclosed in one of the smaller RTL2832 tuner dongles, which is the “Worlds Smallest HF Receiver”. Marty writes

This is a non-preamplified model receiver but has EXCELLENT performance for what you get.  It has become my most favorite receiver to use here in the shack because of its size and performance – not that I don’t like the preamplified models of my receiver line.  I just like how well it performs for such a small receiver – it’s truly unique.

This Small Form Factor Radio has an operating frequency of 100kHz to 54 MHz allowing the 6m band coverage that everyone was so rabid about.  The entire design is predicated on using 0402 SMT parts except for what can’t be squeezed down (the crystal oscillator and the SA612 mixer).  I have built 6 of these radios and have 3 on hand now for sale now.  They retail out at $70.00 because they are HARD to manufacture and are hand-assembled and tested.

If you are interested you can contact Marty at martywittrock_AT_gmail_DOT_com.

Below are some images. The first image shows the size of the worlds smallest HF receiver, the second shows an example of expected HF performance in HDSDR, and the third image shows the upconverter circuit layout.

Worlds Smallest HF Receiver

AM Band Reception from Smallest HF Radio

Smallest HF Radio 15mm x 15mm Circuit Plot

Production 4 Model

In addition to creating the worlds smallest HF receiver, KN0CK has also been working on his “Production 4” model, which has the same performance as his “Production 3” model posted earlier, but is capable of being installed into any RTL2832 DVB-T radio.

This model is now released and will be available for sale at easy-kits.com when “Production 3” is sold out.

Marty writes

The latest production I have is termed as ‘Production 4’ and became a hybrid of my ‘Small Form Factor Radio’ and the existing ‘Production 3’ design.  I took the best of both worlds and then modified the low pass filter to operate into 6m like I did on the Small Form Factor Radio, too.

[The ability to install into any RTL2832 DVB-T radio] was the reason for this final [Production 4] spin – so it could go into the DVB-T radios that incorporated the goofy through-hole capacitors they used in their power supply design (and also cheaper to sell).

I’ve incorporated this new HF upconverter design into the RTL2832/820T model radios, too, and while the performance of the DVB-T stick is not wonderful, it works pretty well with the HF Upconverter I have designed and installed in it.

I must tell you that the RTL2832/820T model radios are the cheapest out there and have 3 goofy through-hole caps in there that made it impossible to install my ‘Production 3’ model HF Upconverter into that radio….Until Production 4 came along… 🙂

The three images below show the circuit layout, upconverter and RTL-SDR dongle together, and upconverter circuit by itself.

Production 4 Board Plot

Receiver Opened Up

Upconverter Board by Itself View1

Future Models from KN0CK and Transmit Capabilities

For the future plans of his worlds smallest HF receiver Marty writes

At this time, I’m going to respin my ‘Small Form Factor’ board to solder the crystal oscillator directly on the board like the Production 4 models do (for awhile, I didn’t have a circuit pads for that oscillator and had to hand wire those on the back of the ‘Production 3’ boards.  Production 4 cleared that issue and now I’ll clean up the Small Form Factor radio the same way.

Finally, Marty talks about his plans to create a transmit (TX) modification to the dongle. He writes

[I] am working toward a transmit side app for this stick – – target is December to make that happen.  I would keep the ‘Production 3’ form factor and then put the TX stuff on the back side of the board.

We look forward to Marty KN0CKs future work, which is likely to be posted about first by KF7LZE, a friend of Marty’s on KF7LZE’s popular blog.