RF DIODE DETECTORS.

Simple diode detectors are not linear. There is a small forward voltage drop across the diode, and this voltage varies with diode current. Even if we allow for a fixed voltage drop across the diode, measurements will not be accurate at all power levels. At 1mA of diode current, the drop across a Schottky signal diode will be about 0.3V. At very low diode currents of around 1µA the drop will be much lower, typically about 0.1V.

This is not too much of a problem at higher power levels because an error of a fraction of a volt is quite a small percentage of a total peak voltage of several tens of volts. We can choose to ignore it or partially compensate by adding a fixed offset to the measured value. About O.6V for a silicon diode or O.3V for a Schottky signal diode such as 1N5711 or BAT 43 is close enough to be reasonably accurate. However, at very low power levels, the voltage drop is a significant fraction of the peak voltage and will lead to increasing errors as the peak voltage is reduced. For example, if the peak voltage is 500mV (+4dBm, or 2.5mW), a voltage drop of 0.2V across the detector diode would result in a measured peak voltage of just 300mV (0.46dBm, or 0.9mW). This problem gets even worse when the peak voltage input approaches 0.1-0.2V and the detector output voltage is close to zero.

 

There are a number of ways to improve the accuracy of a diode peak voltage detector. We could calibrate the meter by hand to eliminate errors at the lower end of the scale. This approach works well in practice but it is time consuming and requires unique calibration curves for individual diodes.

We could apply a small amount of forward bias to the diode which would reduce the voltage drop for RF signals or we could use a second identical diode as a reference to show us the required offset for a given level of current and diode temperature. It would be possible to apply all three methods to obtain the best possible accuracy but to keep things simple; I will adopt only the last method. Figure 3 shows how an opamp and a second diode can be arranged to compensate for the voltage drop of the detector diode. This circuit is due to KI6WX [2]. When used with a closely matched pair of 1N5711 diodes, this circuit will accurately track the input voltage down to a level of well below 0.1 V (-10dBm, or 100µW). If the circuit is to be used with a single-ended power supply, the opamp input and output voltage range must go all the way down to the negative supply rail. I used one half of an LM358. A CMOS input type like the CA3140 would be capable of even better performance.

Homemade HF Antenna Balun

 

A balun is a device that is used at the feed point of a balanced antenna when an unbalanced feed line is desired to feed the antenna. Balun is a contraction for BALanced to UNbalanced. A common example of where a balun would be desired is at the feed point of a dipole antenna when a coaxial transmission line is used. If a balun is not used it is possible for common mode currents to be present on the feed line. The effect of this could be undesirable if the directional properties of the balanced antenna are to be maintained.

Since the feed line usually leads into the shack RF could be present in the shack to create RFI as well as the possibility of receiving excessive amounts of RFI from indoor noise sources. It is often found that a balun is not necessary and everything works just fine feeding the balanced antenna directly with coax cable. When this is possible it may be found that the feed line is an odd multiple of 1/4 wavelength. In this case the transmitter end of the feed line is usually grounded and up from this point on the coax 1/4 wavelength or a multiple thereof will appear as a high impedance. When this high impedance point occurs at the feed point chances of common mode currents are low. Rather then take any chances it is often recommended to use a balun.

There are several different kinds of baluns. Some provide a 1:1 impedance ratio while others can provide 1:1.5, 1:4, and many other impedance ratios. A 1:4 ratio balun would come in handy if you were feeding a folded dipole (200 Ohms) with 50 Ohms coax. For a 1:1 ratio a balun can be constructed using the feed line itself by simply winding about five turns of the feed line around a 2" diameter piece of PVC. I preferred a 1:1 ratio balun that I could easily move from one antenna to another by simply unscrewing the coax.

My balun uses AWG 12 enameled wire trifilar wound on a 6" X 1/2" piece of ferrite rod. 7 turns are tightly wound around the electrical tape covered rod. The free ends of the windings are connected as shown below in the schematic. The whole balun is installed in a 10" piece of 1-1/2" schedule 40 PVC pipe. A SO-239 coaxial connector is installed in the bottom end cap with #4 stainless steel hardware. An eyebolt is installed in the top end cap. The antenna post consist of #10 stainless steel hardware mounted on opposite sides near the top of the PVC pipe.

First I drilled all of the necessary holes, including a drain hole in the bottom end cap, and then painted all of the PVC pieces with olive drab paint the protect from the elements. Next the balun was connected to the SO-239 connector and then the pipe was slid over the balun and cemented in place with PVC cement. At this point the balun was connected to the antenna binding posts. Then the top end cap was installed with PVC cement. I tested the balun by attaching a 50 Ohms termination to the antenna posts and my MFJ-259B via coax to the coax connector on the bottom. The 50 Ohms resistive impedance was reflected back through the balun with little reactance throughout the HF spectrum. Since the design was based upon a tried and true design I am confident that it performs as expected as far as choking off currents.

I found this balun really easy to build and should easily handle a large amount of RF power as long as the SWR of the antenna remains low. A purchased balun may only cost a little more then my homemade version but I had the parts on hand and it was fun to build.

 

Homebrew Antenna Tuner

 

 

Steve Yates - AA5TB

For years I've thrown together simple antenna tuners to get me on the air when needed. Whenever I would go to a sidewalk sale or hamfest I would pick up any components that might someday come in handy to make an antenna tuner. Commercial antenna tuners are very expensive and often of questionable quality. They are so very simple to make that I find it hard to justify purchasing a store bought unit. I realize that good antenna tuner components can be quite expensive when purchased new if you are even lucky enough to find a supplier. If you are not in a hurry all of the necessary components can be found used for very reasonable prices.

There is nothing special about the antenna tuner described on this page. It is a simple T-network and I have found that it can match any unbalanced antenna system I've ever put on it. I've always used PI-networks but component values can become unwieldy at the lower frequencies for such a network. Below are some more photos of my T-network.

Front View

Rear View

Inside View

The components were all found on the surplus market and even though it looks very old it's really only a few years old. The metal enclosure is probably 50 years old even though it had never been used when I purchased it at a sidewalk sale. The roller inductor is silver plated and incorporates a very good turns counter. It is WWII surplus and I had been saving it ever since I came across it as a kid. The two capacitors are 500 pF each and I removed them out of a defunct automatic antenna tuner that was once used at a shore station for ship to shore communications. I purchased all of the insulators from a local surplus outlet. Even the knobs and dial plates are ancient surplus. After all is said and done, I have about $10 invested in this tuner but to purchase one of similar quality the price would probably be about $300.

I don't have plans for you to follow in order to replicate this antenna tuner but as you can see via the schematic it is very simple. I would suggest obtaining the components first and then design everything around them. Use short, fat conductors if possible to interconnect the components. Ther required capacitor plate spacing is determined by the transmitter power and the impedances involved. For 100 W or less the plate spacing of most available air-variable capacitors is probably adequate. The capacitors that I used here have a maximum capacitance value of 500pF each but I probably could have gotten by with 250 pF units. For the inductor try for around 25 µH if you plan to use the tuner on 80 m and maybe 160 m. The larger the inductor's conductor, the better. If you can't find a roller inductor then a tapped inductor can easily be made assuming an adequate switch can be obtained. Of course for simple open-air designs, a wire and alligator clip will suffice.

If the power you plan on using is at QRP levels, say less then 5 W, then the components can be greatly reduced in size. The polyvaricons that are often found in less expensive AM radios can be used for the capacitors and a small tapped coil wound on a toroid will suffice for the inductor. It should be noted however, that the efficiency of this antenna tuner is inversely related to the losses in the inductor. Therefore, even though a small inductor will not burn up at QRP levels, a dB of loss is a dB of loss at any power. What I am getting at is that at some impedance ratios, the RF currents in the coil can become relatively high. In these cases the losses within the inductor can become high unless care is taken to keep the Q of the coil high. This can be done by making the inductor out of the largest conductor possible and by making sure any contact resistance, such as the alligator clip, is kept at a minimum. It should be noted however, that this contact resistance will usually swamp out the RF resistance of the conductor. In other words, if you are using an el cheapo alligator clip or tap switch, there isn't too much sense in going to a 00 AWG conductor ;-)

I only created this page to hopefully encourage others to try and build their own antenna tuners instead spending good ham radio budget on an expensive commercial tuner. I apologize for not having exact instructions but this is an easy project to do on your own with the components that you have available.

QRP Power Meter and Dummy Load

 

Steve Yates - AA5TB

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Last Update: June 14, 2010

Many years ago I acquired a antique field strength meter and probe kit that did not function. However, the meter had a fast response time and a good enclosure and I thought it would make a good piece of test equipment. I designed the following circuit around what I had and it has worked out well for me.

The schematic below is of my QRP power meter and dummy (50 Ohm) load combination. The 50 Ohm load consist of resistors R2 through R5. The four 200 Ohm resistors in parallel combine to make 50 Ohms. I used four resistors because this minimizes the component lead inductance of the resistors as well as distributing the power dissipation. The meter is simply a current meter with a known internal resistance configured as an RF voltmeter. D1 rectifies the RF voltage across the load resistors and C1 charges to the peak of this rectified voltage. The capacitance of C1 is chosen so that the time constant of the RC circuit consisting of C1, R1 and the meter's resistance is long compared to the RF cycle. R1 is chosen so that when the RF power applied is 5 Watts the meter reads full scale.

The calculations are as follows:

The internal resistance (Rm) can be found by constructing the simple circuit below and performing the following calculations:

Adjust R until the meter reads it's full scale value. Be sure to start with R at it's maximum value to prevent damage to the meter. Solve the following equation to find the meter's internal resistance.

The formulae below are to convert the reading on the microampere meter to watts and back. Please note that the possible error caused by the diode's nonlinear response below about 100 mW has been ignored. The scale in the region of tens of milliwatts could be calibrated against a known calibrated power meter or signal source if desired. For more information regarding the very low power measurements with a diode detector you may want to check out "Square Law Diode Detectors in 50 ohm Systems" presented by Glen, VE3DNL.

HF POWER AMPLIFIER

 

 

In my prototype I used IRF840 in the final. Most of the power FET are designed for high voltage operation. At lower operating voltages they saturates quickly limiting the output power. I had given 120 V for IRF840 it takes 1 Amp at peak. Gate voltage is fixed at 1V. Heavy head sink is essential for IRF. My heat sink measures 30 cm * 6.5 cm. Use mica insulator and heat sink compound for fixing IRF.

You can directly replace IRF840 with many of the power FET like IRF830, IRF530, IRF540 etc... When using a different IRF, supply voltage should be changed to less than half the maximum drain voltage (Vds). A zener diode rated slightly higher than the twice the supply voltage connected across drain and source can prevent drain source breakdown. Peak to peak gate voltage of magnitude more than 20 Volts can destroy the FET instantaneously. Two numbers of 15 Volt zener diodes are used to keep gate voltage swing below 20 Volts. Specifications for some of IRF series are given below.

End Fed Half Wave Antenna

 

The End Fed Half Wave Antenna (EFHWA) is fed at a voltage node via a parallel resonant circuit against a ‘short counterpoise’, it is a favourite of backpackers and outdoor types.  It can be considered as a half wave dipole that’s end-fed at a voltage node rather than the current node, as is more usual. This is a very handy arrangement for portable QRP work.
EFHWA Link: http://www.aa5tb.com/efha.html

End Fed Half Wave Antenna by AA5TB
http://www.aa5tb.com/efha.html

I suspect that nothing new or radical has happened in the field of radio aerials in a VERY long time, like at least many tens of decades.  Most of the new wonder aerials are really a con.  Choke off the feed-line and then see how good they really are.  Prime among the baddies is the CFA.  It doesn’t really work, at least if you place a choke in the feed-line.  With any real aerial, there should be minimal radiation from the feed system… so a choke should really make no difference at all, but for the CFA it does!  The CFA is not alone, there are others.  The popular G5RV is another design with a radiating feed, deliberately so, but of course G5RV planned it that way.  He wasn’t cheating… merely being a bit devious, to make it multi-band

Lots of stuff to pass on to my fellow radio club members, most of whom are of the  ‘if it’s not expensive, it can’t be any good’ school of thought when it comes to aerials. Nothing of course could be further from the truth!  Aerials are one area where it makes a lot of sense to build our own." Website of GM1SXX - www.observations.biz

Thanks for your email Allan. It's a good idea to point out that an antenna could be pressed into use on odd multiples of its resonant frequency, hence a 3.6MHz antenna for 80m could be useful near the 30 metre, 10.1MHz, band - near to the third harmonic of 3.5 MHz although, as you observe, the radiation pattern may be quite distorted from the traditionally expected dipole pattern and be more petal shaped. The same goes for a 7.1 MHz antenna for 40m being usable on its third harmonic of 21.3 MHz for the 15m band - a 40m dipole being three half waves an the 21 MHz band.

I have not experimented with a full size 80m dipole, but I would guess that it might be useful at 5 times 3.6Mhz in the 18 MHz / 17m band?
The point made about feeding a familiar dipole at the current node rather than the voltage node is obviously very important and, I imagine, sometimes overlooked.
PLANS: Download the pdf plans produced by G0KYA here > More from G0KYA here: http://g0kya.blogspot.com

End Loaded Dipole

End loading can also help reduce the size of antennas, particularly useful for dipoles used on the  80m and 160m bands. An end loaded dipole will produce an antenna that is H shaped. There are several commercial designs available produced in designs that cover a single band and others that cover multiple bands. The version shown below is only 3 metres tall so will be suitable for very unobtrusive, low profile use. It is the ProAntennas Multi-band I-PRO: 20m 17m 15m 12m 11m & 10m which uses a capacity hat with some loading at the centre. http://www.proantennas.co.uk/

Other similar antennas were available from Force12 Antennas in the form of, amongst others, the Sigma 5 and Sigma GT5. The Sigma design used T-bars at each end of the vertical dipole for loading technique and off-center loading coils.  http://www.force12inc.com This was supplied supplied in the UK by Vine Antennas at one time http://www.vinecom.co.uk . Transworld Antennas also have produce antennaa using a similar concept - the TW2010 Adventurer and Backpacker http://transworldantennas.com

K9AY Notes that: "I have come to the conclusion from my experiments, readings and observations, that a capacity hatted vertical dipole, a few feet over ground, is less compromised than a 1/4 w/l vertical of the same height fed against a less than perfect ground. Let's face it, most amateur's ground systems are mediocre at best. Also, the dipole is easier and cheaper to rig, and is two dimensional..Very important in my situation, as I cannot run out radials on my neighbours property.  Or, to quote W4RNL.."Since only a handful of hams can ever have 160-meter antennas high enough to yield a low angle DX signal, more practical are vertical arrays such as yours.  Vertical dipoles with hats (or Tees) save a plethora of wire needed by monopoles." http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=7466

Information by K9AY

Information by K9AY

Interesting concepts from K9AY

An effective multi-band "4 Dollar Special" by Joe Tyburcy - W1GFH

 
Joe Tyburczy, W1GFH provides some sensible insight and advice, he writes: "I am a big fan of "balanced line" (twin lead, open wire line, etc.) vs. coax. By using balanced line and a tuner you can have one, single-element antenna that works well on all bands. You can't do that as easily with coax. The basic "W1GFH $4 SPECIAL" shown below is a variation on the type of versatile skyhook I've been using for years......Now at this point, some of you may be looking at the diagram and muttering, "Jeez Joe, that's just a dipole fed with twin lead and used with a tuner". Well of course it is. Virtually all antennas are "di-poles" (i.e. "two sides") in some form or another. This one just happens to be made from low-cost materials......I won't go into the theory here, but trust me: balanced feed line, properly used, does not "leak" RF and is less lossy than coax. I've tried the commercial 450-ohm ladder line, but prefer 300-ohm TV twin lead, and the cheaper the better. Radio Shack TV twin lead is ideal. Home Depot has some good stuff, too. Forget all the obsessive junk about standing waves, impedance and velocity factor. What you really need to concentrate on is getting an interesting set of antenna insulators."

Read Joe's excellent article in its entirety here: http://www.qsl.net/wb1gfh/antenna.html

4 Dollar Special by W1GFH
http://www.qsl.net/wb1gfh/antenna.html

The Norcal Doublet

 

 

The Norcal Doublet Antenna:  http://www.norcalqrp.org/norcaldoublet.htm

The Norcal Doublet is a simple antenna that is 44 feet (13.4 metres) long, 22 feet (6.7 metres) per side. The Norcal pages report "...that the antenna would have basically the same radiation patterns on all bands from 40 - 10 meters. This would be very handy to have for field operation..... You will need the following materials: 50 feet of 4 core stranded computer cable; 1 #0 Fishing Swivel; 1 Cable tie; 2 pieces of fishing cord."
The antenna can be hung from trees or cheap telescopic 'roach' / Sota poles. Doubling the size would allow operation on 80 metres and even 160 metres by shorting the twin feed together at the transmitter end and feeding it against a good earth as a 'Marconi' type antenna.