Building a 100watts RF dummy load using an RF resistor

Building an 100watts RF dummy load

RF dummy load resistor can now be easily obtained from majority of popular online shops so obtaining one and building your own dummy load would not be a problem nowadays. So what is an RF dummy load?, a dummy load is a device used to simulate an electrical load, it is used to simulate an antenna in an RF system. A transceiver can be tested and configured without radiating a radio wave by using a dummy load. So if you are playing around with radio transceivers an RF load is a nice addition to your set of tools.

Materials

1. RF Resistor rated @ 50ohms/100Watts
2. 1×1 Aluminum square tube
3. Suitable aluminum heatsink (I’m repurposing a GPU heatsink from an old video card)
4. SO239 connector
5. Cutting tool
6. Soldering tools
7. NanoVNA, SWR meter and coaxial cables for testing.

repurposing and old gpu heatsink

For my build I cut a 2cm x 7cm aluminum heat sink from an old video card and attached the RF resistor on top of it and place it inside a 1 x 1 inch aluminum square tube. I cut a mounting hole on the heatsink and the aluminum square tube for mounting the heat sink and RF resistor. An SO239 feed point was used to connect the center tab of the RF resistor by soldering it directly on the center pin. The ground tab of the resistor is secured by a screw on the body of 1 x 1 aluminum tubing that serve as a holding case for my dummy load.

I tested the frequency response of the dummy load using the NanoVNA on both the 2m VHF and 70cm UHF amateur band with very good results. SWR on VHF is 1.07 and 1.2, 49 ohms to 57ohms impedance. The resulting impedance is negligible when using an analog SWR meter SX400.

The finished dummy load can easily handle 65watts transmitter power without over heating on short duration testing using my Yaesu 2900R transceiver. I also used a QYT KT8900D transceiver to test the power output of the radio at the UHF side.

Have a look on the video at my youtube channel for an overall build instruction video and testing.

Replacing NanoVNA toggle switch with more rugged push button tactile switches

NanoVNA toggle switch replacement

NanoVNA toggle switch replacement with push button tactile switches

NanoVNA devices is generally well built in my opinion except for its toggle switch to navigate the menu system and move the cursors around the screen. The toggle switch used on my device is unfortunately too flimsy it just broke after using it for a while. The newer devices comes with a more reliable toggle switch, however ordering it online on this part of the world is a bit too costly for just a single switch replacement. So instead of ordering it online and wait for it to arrive in the mail box I’d rather use a more robust push button tactile switches as replacement.

Solder the 3 switches together
Soldering the 3 switches together and tie one pin of each switch to the body to serve as ground terminal.

Drill an equally space holes on the front cover to accommodate the switches to peek on the holes. This will serve as the switch mounting position for easy push button navigation. Then solder the switches to the PCB contact points as shown on the diagram above.

Drilli holes on NanoVNA front cover

Put back the NanoVNA cover plate and it’s ready for testing.

NanoVNA with Push Button tactile switch testing

Restoring Diamond F22A antenna

Restoring Diamond F22A amateur radio antenna

Diamond F22A antenna is a two element 2m 7/8 wave high performance FRP (Fibre Reinforced Placstic) radome vertical antenna. This type of antenna is inductor loaded (it has loading coils) to bring it to resonance a small capacitor is needed at the base of the antenna to tune it to its designed frequency.




The original specifications states that the antenna is designed on the amateur frequency 144-148Mhz @1.1:1 SWR at the center frequency 146Mhz.

Specifications

Frequency 144-148Mhz
Gain 6.7dB
Impedance 50ohms
VSWR Less than 1.5:1
Max. input power 200W
Max. wind resistance 50m/sec (112.5MPH)
Mast diameter accepted 3-62mm (1-1/5″ to 2-2/5″)
Length 3.2m (126.0″)
Weight 1.3kg (2.9lbs)
Connector UHF Female
Type 7/8 wave two-element

Download the original antenna manual here

Usual Problem

The usual problem of this antenna before if it stops to work are the frequent symptoms of intermittent VSWR and the sudden high VSWR read out after some time. This usually indicates that the loading capacitor is now broken and must be replaced. However another problem encountered by majority of users of this antenna when the capacitor fails is the availability of the replacement capacitor which are often not easily accessible on the market. The capacitor value for this type of antenna is rated at 6pf, but you can use a capacitor in the neighborhood of about 15-to-20pf and the antenna will still work reasonable well. But for the purist you may stick with the 6pf and just be happy with it :). The usual problems of capacitor availability can be fixed by substituting a portion of coaxial cable to replace the capacitor. You need to check the coaxial cable datasheet and compute the nominal capacitance per foot or per meter to estimate the length of the coaxial cable you need to cut.

Download the RG174 Datasheet here

Nominal Capacitance

In this case we have 31.08 pf/ft for the RG174 to compute for the length of the coax we need, convert ft to cm.

1 ft x 12 in 1 ft x 2.54 cm /1 in = 30.48 cm

So to estimate the length of the coax we need, divide

31.08pf/30.48cm = 1.036pf/cm, so if we need 15pf of capacitance that is

<= 15cm coaxial cable length.

Feel free to cut about 15cm of RG174 coax and just trim it slowly when tuning. In this case I have use about 15pf less than 2cm for the soldering points so that is about 13pf.

If you need more accurate capacitance value you may also use a capacitance meter to measure the length of the coax before you solder it to the antenna as capacitor replacement.

Frequency response of the antenna before and after the coaxial cable replacement




Enabling battery status indicator on NanoVNA

NanoVNA with battery indicator

Battery status indicator on the NanoVNA

Most available NanoVNA on the market has a working firmware that works from 50Khz to 900Mhz this version of the firmware doesn’t have the battery status indicator enabled. At the time of this writing there is already a version of firmware that can be flashed on the NanoVNA device to indicate battery status. This extends the frequency scanning to 1500Mhz although it may not be fully usable without a hardware upgrade. It contains experimental TDR function at this time originally released by edy555 on Github for NanoVNA the firmware release version is 0.2.2 inside the releases folder the files are provided as hex and bin need to be converted in DFU before you can use it in DfuSEdemo software unless you have a programmer for STM32 micro controller. I have converted the hex file to DFU and you may use it to flashed your device. If you don’t know what you are doing do it at your own risk the software is provided as is.




Download the files here. The original version of the files can be downloaded here https://github.com/ttrftech/NanoVNA/releases

Two steps to enable battery indicator

Step 1:

Add a missing diode on the board of NanoVNA. The diode is near the battery terminal marked ad D2 this was missing on NanoVNA board you need to add one. This was use to enable the actual status of the battery if it is fully charge or empty.

missing diode on the nanovna board

Use a suitable diode for this to mount on the NanoVNA pcb, edy555 recommends 1N4148WS or B5817WS or any small diode however I don’t have those ready so I looked into my tool box (junk box) and found PG05FSESC a TVS diode for ESD protection in portable electronics. Download PG05FSESC datasheet here.

PG05FSESC diode
PG05FSESC diode a TVS single line diode for ESD protection marked as 5S

Test the polarity and solder the diode on the board.

Testing diode polarity

soldering the diode on nanovna board
Soldering the diode on the NanoVNA board

Step 2:

Flashed the DFU firmware above to your NanoVNA device I have not include the actual flashing of the firmware here, but you may watch the instructions on how to do it from my youtube channel from this link: https://youtu.be/XBOTRcZI1qQ

When flashing the file you must first use the DMR-CLEAR_MEMORY_DFU.dfu file included from the downlink above before you flash the 0.2.2.dfu file.

After flashing the firmware you just need to re-calibrate your device and it’s ready again for normal use.

Video instructions and testing of the NanoVNA after the procedure




How to change NanoVNA Firmware

The NanoVNA kit

The NanoVNA

The advantage of NanoVNA being open source is that people in the community are building software and firmware that are both feature rich and useful for the community of users. The NanoVNA uses STM32 micro controllers 32bit arm cortex MCUs. Loading a new firmware on the NanoVNA requires DfuSE software to load the DFU firmware available for download on the creator of the device Github pages, a quick search on google will let you download the software that we need to use in programming the NanoVNA device. I’ll put the software that I used here accessible on my Google Drive however it is advisable to get the files directly from NanoVNA creators pages as those are most up to date.




Steps in updating the firmware.

1. Prepare the NanoVNA to start in DFU mode.
2. Make sure that the device drivers are properly installed.
3. You need 1 open USB port for downloading the firmware on the NanoVNA.
4. Install DfuSE programming software.
5. You need access to the DFU firmware files.

Preparing the NanoVNA to start in DFU mode. I installed two pins on the the NanoVNA board marked as BOOT and VDD so I can easily short those pins with a clip or a twizzer to activate DFU mode.

Preparing the NanoVNA to start in DFU mode
Preparing the NanoVNA to start in DFU mode

Install the device drivers for the NanoVNA so that it can be detected by the DfuSE programming software.

STM Device in DFU Mode driver
STM Device in DFU Mode driver. When installed properly it can be found under the Universal Serial Bus Controllers node. When properly installed the NanoVNA can be detected by the programming software.

When the device driver is not properly installed the software will not detect the NanoVNA.
No drivers installed or incorrect driver

A properly installed driver will enable the programming software to detect the NanoVNA device automatically.

Correct NanoVNA driver installed
Correct NanoVNA driver installed. It will enable the programming software to instantly detect the NanoVNA when loaded.

Finally put the NanoVNA in DFU mode and start changing the NanoVNA firmware. Short the terminal marked as BOOT and VDD on the NanoVNA, connect the device on one empty USB port and turn on the device. This will activate DFU mode and the device is ready for programming.

Changing NanoVNA firmware
Changing NanoVNA firmware put the device on DFU mode

Download all the softwares here from this link.

Here’s the complete video for changing the NanoVNA firmware.




3×5 Cross Yagi tear down for easy storage

This 3×5 Cross Yagi can be totally tear down for easy storage. After the procedure we are left with just a few pieces of the antenna elements are neatly tucked inside the boom. Gamma match, tuning stub, phasing harness, gamma match bracket, mounting bracket, and screws can be put inside a separate plastic bag for safe keeping.

This is the complete tear down video linked to my youtube channel. Take time to watch and enjoy.

Disassembled 3x5 Cross Yagi
Dis assembled 3×5 Cross Yagi for storage




Pencil Torch conversion to continuous flow butane soldering system

Pencil Torch Butane fuel

Pencil TorchPencil Torch is a good tool for soldering materials requiring extra heat. However care must be taken to successfully use it on electronics soldering work. The downside of the stock pencil torch is that from time to time the flame becomes intermittent due to the soot forming from the torch burner mainly cause by butane impurities and largely because as you use it longer than necessary the air pressure on the canister becomes weak. The cartridge can be refilled at the bottom of the torch through its pin charging system much like that of a car wheel. You poke the butane canister and press it for about 10 secs for a complete charging using a pumping action. This pushes the charging pin on the torch cartridge to allow fuel charging. To turn on the pencil torch use a lighter and slowly open the valve knob until you can hear some air escaping and light is using a lighter. A complete charge will usually allows you for about 2 minutes of working without interruption however when butane charge is slowly consumed you may experience intermittent operation. The reason I decided to convert this to a single continuous flow soldering system using a broken flame torch to be use as feeder system.

Butane Canister



Materials for conversion

Pencil Torch
Flame Torch (Feeder system)
3/16 Copper tubing
Moldable epoxy
Tube cutter
Coaxial Cable (Insulator only)
Tube clamps
Silicon tubing
Shrinkable tubes

Pencil torch conversion to continuous flow soldering system

1. Remove the charging pin on the pencil torch
2. Cut about 1.5 inch of copper tubing to be use as extension for both the feeder and the inlet on the pencil torch
3. Insert the copper tubing on the pencil torch charging port and secure it with moldable epoxy until it cures
4. Do the same on the butane feeder system
5. Once the epoxy has completely cured attach the silicon tubing to the torch fuel inlet and the feeder system by inserting about an inch of coax insulation to the silicon tube.
6. Secure this connection with the tube clamps
7. Finished the conversion process using shrinkable tubes on joint for a more professional look.



UHF Yagi antenna 435-445 frequency sweep analysis

Frequency Sweep Analysis UHF Yagi 435-445Mhz

This is a short video demonstrating the performance of N1201SA RF Vector Impedance Analyzer in comparison with Diamond SX400 SWR meter on individual frequency check across the designed frequency 435-445 Mhz on a UHF 70cm Yagi. The advantage of course of the N1201SA over the Diamond SX400, is it can directly plot the frequency sweep analysis of the antenna very quickly on the LCD monitor. However when it comes to precision both of the devices gives out a similar readings with little variation from the analog display of SX400 as compared to N1201SA digital display.



N1201SA VNA – VSWR read out compared to Diamond SX400

SX400 vs N1201SA

N1201SA Vector Impedance Analyzer

The N1201SA is a Vector Impedance Analyzer or commonly known as antenna analyzer from AAI technologies in China. AAI stands for Accuracy Agility Instrument. The strong point of this antenna analyzer as compared to other brand is the range of frequencies which it can operate from 137Mhz to 2.7Ghz aside from the digital read out and quick performance of the device to display the parameter of interest it also features a frequency sweep analysis with graphing function which you cannot find on most known brands within the same price range. It currently sells at around $148 USD online.

The primary reason I purchased the device is base mainly on good performance review from other users aside from the range of frequencies it can operate which is very much suitable for much of my work. I currently have a Diamond SX400 which is a dual band VSWR meter suitable for VHF/UHF work the only downside is it only measures VSWR, forward and reflected power on analog dial display, with N1201SA VNA it measures most of the antenna parameters I need on digital screen read out, coupled it with antenna simulation analysis software 4NEC2 life is really much easier.



Diamond SX400 VSWR Meter features

So for quick specifications the Diamond SX400

The SX400 measures forward and reflected power and VSWR. Compact size makes meter useful for testing both base and mobile installations.

 Diamond SX400

• Illuminated meter
• Switchable r.m.s. or peak power
• Measures forward, reflected & VSWR power
• 6″W x 2″H x 4″D, 2 lbs.
Specifications:
Frequency: 140-525 MHz

NS1201SA Vector Impedance Analyzer features

N1201SA series is the handheld radio frequency vector impedance measurement analysis instrument, easy to use, simple operation. Built-in high capacity lithium ion battery for mobile and outdoor use.

N1201SA Vector Impedance Analyzer

Specification:

Working frequency: 140MHz~2700MHz
Actual frequency : 137.5~2700MHz
Stepped frequency: 1kHz
Display: 2.4″ TFT trdp
Resolution ratio: 320 x 240(QVGA)
Battery capacity: 2000mAH(7.4Wh)
Power consumption: <1.5W Charge current: 400mA Charge port: USB Auto power off can be set 5-60minutes. Measured parameters: Resistence, Reactance, Standing wave, S11 Resolution ratio: 4 number Frequency accuracy: about 5ppm Connector: SMA-K Measurement range: Impedance: 0.1~1000 Standing wave: 1.000~65 S11(dB): 0dB~-60dB Working temperature: 0~40℃ Atmospheric pressure: 860hPa~1060hPa This instrument has four interfaces: single point measurement, scanning, system information, correction and calibration The default boot into single-point measurement interface.

Measurement comparison on VSWR Readings

With both of the device specifications presented we are ready to proceed and compare the actual measurement on VSWR readings and compare the accuracy of the read out against each other. Please see the video below of actual comparison of VSWR read out on both device.



3 Elements Yagi nominal range at 5 watts

Point to point Sariaya to Atimonan 45kms

This is a testing video for 3 Elements Yagi (Lightweight End Mount) using a portable HT Cignus UV85 (A Baofeng UV5R variant). I’m testing from my present location at Maligaya, Atimonan Quezon climbing a hilly location near my QTH. The terrain is a mountainous area with a bowl like topography with lush vegetation. I managed to contact the station from Sariaya, Quezon about 45kms from Atimonan point to point.

Testing nominal range using HT @ 5watts



This Yagi is designed for 146Mhz as center but easily covers 144-148Mhz with similar pattern across the operating frequency this Yagi is designed using 4Nec2 antenna simulation software. Antenna boom measurement is the same as the 3 Elements Yagi designed for 145Mhz except for the elements which are cut for 146Mhz as center frequency.
2m 146Mhz Yagi

Like most of my antenna design this is lightweight collapsible for easy keeping, and end mount. The advantage of end mount design is obvious as there is no pattern distortion on the antenna unlike the mid mount configuration.

Google Map Highlighting Sariaya Quezon

Google Map highlighting Sariaya Quezon



Point to point distance of Sariaya Quezon to my location in Atimonan, Quezon
Point to point Sariaya to Atimonan 45kms