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.

 

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.

Loop Antennas - Delta Loops and Square (Quad) Loops and more

 

Delta Loops for HF  -  "You'll love lower noise and relative gain over a dipole"
http://w5sdc.net/delta_loop_for_hf.htm
One Stealthy Delta - This HF antenna keeps a low visual profile while attracting plenty of attention on the air.
An excellent and amusing article by Steve Ford, WB8IMY
http://www.sgcworld.com/Publications/Articles/237qst0502.pdf
Random length multi-band delta loop antenna – A good antenna for when a dipole isn't enough by KC8AON
http://www.i1wqrlinkradio.com/antype/ch10/chiave1827.htm
An Easy to Install Vertical Loop for 80-6 Meters by John Reisenauer, Jr. KL7JR
http://www.hamuniverse.com/kl7jreasyvertloop.html
40m-10m DELTA LOOP ANTENNA - GU3WHN
http://www.rsars.org.uk/ELIBRARY/ANTENNAS%20DOCS/40m-10m%20%20DELTA%20LOOP%20ANTENNA%20-%20GU3WHN%20iss%201.3.pdf
M0PLK Multiband Delta Antenna - By Arthur M0PLK (SQ2PLK)
http://pdxa.one.pl/articles.php?article_id=17  available at http://ham-radio.urbasket.eu  and  http://www.vpa-systems.pl/
H5ANX Mk4 Delta Loop Design by Sajid Rahim
http://www.eham.net/articles/10738
Multiband H.F. Delta Loop by IW5EDI:
http://www.iw5edi.com/ham-radio/?dl2hcb-multiband-delta-loop,28
SGC Stealthy H.F. Delta Loop:
http://www.sgcworld.com/Publications/Articles/237qst0502.pdf
KL7JR Easy H.F. Delta Loop:
http://www.hamuniverse.com/kl7jreasyvertloop.html
H.F. Loop Antenna from Radioworks:
http://www.radioworks.com/nloop.html

W6ZDO Portable H.F. Delta Loop Project:
http://www.fros.com/KI0GU/w6zodelta.htm
Loop Antenna Notes by "Yukon John" KL7JR
http://www.hamuniverse.com/kl7jrloopnotes.html
Build a Multi-Band Mono Delta Loop for 40, 30, 20 and 15 Meters by Jose I. Calderon (DU1ANV)
http://www.para.org.ph/membersarticles/DU1ANV/Multi-Band%20Mono%20Delta%20Loop%20ant.pdf
DL2HCB Multiband Delta Loop
http://www.iw5edi.com/ham-radio/?dl2hcb-multiband-delta-loop,28
The Delta Loop (Skywire) Antenna - Legends, Theory and Reality
http://dk5ec.de/deltaloop-eng.htm
Loop Antenna notes and ideas from Radioworks
http://www.radioworks.com/nloop.html
Delta Loops by GW7AAV
http://www.cqhq.co.uk/2009_05_01_archive.html
More Delta Loop links:
http://www.i1wqrlinkradio.com/antype/delta_loop.html
Magnetic Loops:
Small Transmitting Loop Antennas (Magnetic Loop Antennas) by Steve Yates - AA5TB
http://aa5tb.com/loop.html

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

W3DZZ antenna by the Maidstone Amateur Radio Society

 

W3DZZ antenna by the Maidstone Amateur Radio Society that adds a dedicated 10 meter (28MHz) resonant element as a 'fan'.

W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society

http://www.btinternet.com/~shaun.scannell/club/w3dzz.htm

Moonraker supply a whole range of wire trap dipoles covering from 2 to 5 HF bands (MTD1; MTD2; MTD3; MTD4; MTD5; MTD6). Diamond also produce trapped wire antennas, the W-721, W-728 and W735. Comet and Diamond each produce similar interesting 5 band wire dipoles that utilize both traps and a fan arrangement - the Diamond W8010 and the Comet CWA-1000. If space really is limited then look out for KZJ Communications (dongo1950 on ebay) - he produces 'Limited Space Inductive Dipoles'. These are inductively loaded and shortened dipoles so they will have reduced efficiency, of course, but are very nicely made, so might be very useful in a tight spot.

To obtain good efficiency and achieve a low angle of radiation, desirable for longer distance DX, a horizontal dipole needs to be installed at a good height - over 20 feet would be desirable and it is quite common to install horizontal dipoles at around 30 to 40 feet above ground level. This might be a problem at some QTH's, it certainly is at mine!

Allan Copland, GM1SXX comments: "The dipole will operate well on the band it has been sized for , if placed at a suitable height, but will also operate as a’ three-half-wave’ aerial at three times the frequency and so on, so it’s not strictly a single band aerial.  An 80M dipole (132 feet typical) will work nicely on 30 metres  (three half waves) but not on 40m (two half waves)… because on 40M the feed-point  is at a voltage node and not at a current node, for easy feeding.  Most aerials are current fed.

The radiation pattern changes when a dipole is not used on its design frequency. The pattern will break up into multiple ‘petals’. This can be either a disadvantage or an advantage depending on what you expect from it.  Since most of us use co-ax, an UN-BAL  should really be used to connect the unbalanced feeder to the balanced aerial, but how many people actually bother? Not many I suspect.  It’s possible of course to use a balanced feed-line  system instead with a dipole and just have a delta match (no centre insulator… none needed).  There are many choices and permutations, but in general, dipoles are centre fed at a point of current maximum (and minimum voltage).

A normal dipole is current fed but of course can be voltage fed instead. This is what’s done in the EFHWA or Fuchs aerial where a resonant half wave wire is fed at one end (max voltage / min current) from an L/C tank, against a very short counterpoise wire.

Inverted L - 80 metres to 10 metres

 

A typical Inverted L antenna will be trapped for 40m/80m using a 7.1 MHz trap. It is essentially one half of a W3DZZ dipole so can be accommodated very much more easily into a small plot or garden - especially as part of the antenna is running vertically up a wooden or fibreglass (non conductive) pole. This should allow it to be fitted into quite a small garden such as mine.

The Inverted L is also a very effective aerial because it has the benefit of both vertical and horizontal radiation. While Inverted L's might make good TX aerials, like ground mounted vertical aerials they can be quite noisy on RX.

The Inverted L is extremely easy to 'home brew'. Spectrum Communications can also supply the complete aerial as shown below. It should give excellent performance on 80m and 40 metres, with 20 metres also being good but allowing use on 15m and 10m and possibly one or two of the WARC bands: