Let's build a 100 and 20 watt Dummy Load!

Above Image: Large RF dummy load for shortwave transmitter at the Moosbrunn transmitting station of the Austrian Broadcasting Service, Moosbrunn, Austria. It uses a sodium hydroxide solution to absorb the radio energy, which is circulated through the radiator (center) to cool it. It has a capacity of 100 kW.  - Source:  Wikipedia

At some stage of your amateur radio "career" you will need a Dummy Load.  Now what is a dummy load and what is it used for?

A dummy load is a device used to simulate an electrical load, usually for testing purposes. In radio a dummy antenna is connected to the output of a radio transmitter and electrically simulates an antenna, to allow the transmitter to be adjusted and tested without radiating radio waves. A dummy load is connected to the output of an audio power amplifier to electrically simulate a loudspeaker, allowing the amplifier to be tested without producing sound. Load banks are connected to electrical power supplies to simulate the supply's intended electrical load for testing purposes. 

Above Image: Small RF dummy load with capacity of 1 kW at frequencies up to 450 MHz. It consists of a 50Ω resistor immersed in oil to absorb the heat.  Source:  Wikipedia

In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device, usually a resistor or many resistors, used in place of an antenna to aid in testing a radio transmitter. It is substituted for the antenna while one adjusts the transmitter, so that no radio waves are radiated so that the transmitter does not interfere with other radio transmitters during the adjustments. (As no dummy load is ideal, however, some radiation does occur). If a transmitter is tested without a load attached to its output terminals such as an antenna or a dummy load, the power will be reflected back into the transmitter, often overheating and damaging it. Also, if a transmitter is adjusted without a load, it will operate differently as compared with a load, and the adjustments may be incorrect.

The dummy load ordinarily should be a pure resistance; the amount of resistance should be the same as the impedance of the antenna or transmission line that is used with the transmitter (usually 50 Ω or 75 Ω). The radio energy that is absorbed by the dummy load is converted to heat. A dummy load must be chosen or designed to tolerate the amount of power that can be delivered by the transmitter. 

The ideal dummy load provides a standing wave ratio (SWR) of 1:1 at the given impedance.

Veterinarian-grade mineral oil, an inexpensive form of mineral oil, is frequently used by amateur radio operators as coolant in RF dummy loads. 

Above Image:  ZS1I external 100 watt Dummy Load.  Built many moons ago and never bothered to put it in a container of project box.  Still working as it should though!

Now that we know what a dummy load is it is time to put one together.  The dummy load we are going to construct can tolerate 100 watts of RF Power for one minute.  More than enough time to test a transmitter etc. The load uses carbon non-conductive resistors  mounted between two PCB's.

The dummy load we going to construct can be used internal or externally. Here I am talking about a project cabinet.  Your choice.  Mine has been used externally with no cabinet. The dummy load can be made up of  54 x 2k7 2 watt or 48 x 2400 Ohm carbon non-inductive resistors that are sandwiched together, with air spacing of 5 mm, between two identically drilled circuit boards.  The resistor ends are mounted flush with the non-clad side of each circuit board.  Then the leads are soldered to the foil side and cut.  Keep wiring including coax cable as short as possible.  This keeps the ground connection short, which eliminates stray reactances at the higher R.F. frequencies. The finished dummy load, shown in Fig. 1 is capable of dissipating at least 100 watts. 

I also constructed a 20 watt dummy load using 11 x 560 Ohm 2 watt carbon non-inductive resistors.  See images.

Calculation Results:

100 Watt Dummy Load

54 x 2K7 Resistors in Parallel = 50 Ohm

48 x 2K4 Resistors in Parallel = 50 Ohm

20 Watt Dummy Load

11 x 560 Ohm Resistors in Parallel = 50.91 Ohm 

Simple method to test 1:1, 4:1, 9:1 and 49:1 Baluns

 

This article was written to aid new radio amateurs in  testing a homebrew balun.

Using a transceiver, SWR meter and 1:1 balun with resistor to test the balun:

You can test a balun by connecting a simulated antenna load on the output (antenna) side of the balun.  Connect your transmitter ( with an SWR meter) on the input side.  In the case of a 4:1 balun, use a 160 to 340 ohm resistor as your simulated antenna.  (See Fig 1)   If the balun is functioning properly, you should see a low SWR (less than 1.5:1) when you key your transmitter.  This means that the 4:1 balun is matching the nominal 200 ohm load to 50 ohm (200/4 = 50).  If you have a 1:1 balun, you can use the same test. Simply substitute a 40 to 60 ohm resistor for the load.  A 50 ohm dummy load
may substitute the resistor if available.  The SWR should be 1:1 or very close to it. WARNING:  The load resistors must be non-inductive and capable of dissipating the output power of your transmitter for a short period of time.  If you turn your transmitter output down to 10 watts, you'll still need a heavy duty 10 watt resistor.   

I have used this inexpensive old school method for many years but you could substitute your transmitter and SWR meter for a expensive antenna analyzer which will have the same end result.

Testing a 9:1 Balun with a Nano VNA:

 

Images:  (Click on images for larger view.)

I decided to measure the "SWR and Impedance" of the 9:1  Balun with my NanoVNA.  Although this is not the correct way to do it, it gives me some indication that the Balun is working.  Let's call the measurement a fault finding mission.  I used a 270 Ohm resistor across the connecting points and obtained the readings as depicted in the images.

Testing a 49:1 Balun with a Nano VNA:

Images:  (Click on images for larger view.)

To test the balun before adding the wire and counter poise I used a 2K7 resistor on the ground of the coax connector and antenna connection.  The attached NanoVNA image provide info on the results I obtained from the balun.  I am sure the SWR readings will change once I add the antenna wire to the balun.  The height and closeness of objects will also play a roll.

New Series - Building Low Budget Antennas - J-Pole Antenna for 70 Centimeters (433.500 Mhz) - Project 7

I constructed this antenna a few years ago.  It is an inexpensive antenna that is easy to construct. In about one and a half hours time, and less than R100 worth of materials, you can have a great performing omnidirectional J-Pole antenna. This antenna is based on the same idea as the 2 Meter  J-Pole Calculator available HERE. The J-Pole antenna is basically an end fed half wave dipole that uses a 1/4 wave shorted matching stub as an impedance transformer. The J-Pole antenna will yield slightly less than 3 DB of omnidirectional gain.

The material I chose to build the j-pole antenna was 15 + 20 mm copper pipe and a few other fittings used for plumbing. (View the images below for more detail)  Here are the plans: 

(Click images for larger view.)

I cannot remember which measurements I used many years ago to build this antenna and which frequency was used for resonance.  At the time of construction I did not have an antenna analyzer and the only measuring "tool" was an SWR Meter.   I now have a Nano VNA and I decided to revisit this antenna.  I made a modification to the original antenna.  More in this later.

(Click images for larger view.)

The NanoVNA reflected a 1:1 SWR but the antenna resonated on 411.000 Mhz.  On 433.000 Mhz the SWR was 1:2 - 1:3 and the impedance more than 50 Ohm.  The antenna was usable like I constructed it but I want to modify it to see if I can move the frequency to 433.550 Mhz with a 1 : 1 SWR.  I decided to use a piece of soldering wire (30mm) that I added to the 1/4 wave shorted matching stub. I moved the connection points and BNC connector 20mm upwards.  Spot-on the antenna now resonates at 433.500 Mhz with and SWR of 1:1 and the impedance is 50.9 Ohm.  Now that I know that the matching stub must be extended I needed an easy modification.  Well I came up with the modification as shown in the images  below.

(Click images for larger view.)

What does the SWR and Power output look like on the SWR Meter after the modification.  Here are two images and I would say that the two measuring instruments compare fairly well.

(Click images for larger view.)

Finally:  This antenna will be used for the ZS1I SVXLink Reflector UHF Radio and will be installed permanently this coming week.  Although I originally tuned the antenna with a UHF Radio and SWR Meter I must confess that an antenna analyzer is a must have instrument if you want to construct and play around with antennas.  It provides you with so much more information that an SWR Meter.

Christi ZS4CGR toets 'n verskeidenheid van "Rubber Duck" Antennas

Hoe resonant is 'n verskeidenheid van "Rubber Duck" antennas wat algemeen saam met handstelle verkoop word.  OK Chrisi ZS4CGR het 'n paar toetse gedoen.  Let assblief daarop dat die toetse nie in 'n laboratorium gedoen is nie.  Die eindresultate is baie interressant  en gee 'n goeie aanduiding dat sommige "rubber duck" antennas geensins ideale en effektiewe antennas is nie.  Christi se bevindings volg hieronder:

Foto Bo: Nuwe toetse met ’n "ground plane", so het die toets 'setup' gelyk

"Rubber Duck" Antenna VNA toetse - Christi ZS4CGR

Antenna:                       MHz               MHz              MHz

Resonansie:                  Res1               Res2             Res3

UV-5RH                         226                 427

UV-5RA                         211                 434

UV-5R                           204                 427

Diamond RH951S       187                 395

Nagoya NA-771          118                 140               466

PA-NA771                    126                 420

 

Kommentaar: Nie idiale toets omstandighede nie!

Antenna 1 tot 3 het saam met hulle hand radios gekom

Antenna 4 en 5, soos benaam

Antenna 6, het ek saam met n 2de stel UV-5RA ingevoer vanaf E-Bay (net betyds voor die slang-byt-spuit periode)

 

Hier volg nou toets grafieke met ’n grondvlak: (Klik op fotos om te vergroot)

 

Die oorspronklike rubber duck antenna van die UV-5R

Die oorspronklike rubber duck antenna van die UV-5RA

 Die oorspronklike rubber duck antenna van die UV-5RH

Die oorspronklike rubber duck antenna van die UV-9R

 

 Diamond RH951S

Nagoya NA-771

 PA-NA771

 

So, wat sê ons nou vir mekaar:

1) Toets die skroefies agterop die radio aan die bokant om te sien watter skroefies het "grond kontak" met die antenna

2) Terwyl jy praat, maak seker dat een van jou vingers kontak het met die "grond skroefie"

3) Dit is duidelik dat een of ander vorm van "counter poise" ’n groot invloed het op die SWR en ook impedansie.

Ek hoop hierdie toetse gaan help om beter resultate te verkry met hand radio’s.

73 uit Luckhoff. Christi  ZS4CGR