Q can be a valuable indicator of antenna system health
The Q of an antenna can be a useful statistic in assessing whether it is operating as it should.
The Q of half wave dipole antenna system on HF depends to some extent on conductor size, its environment (height, type of ground, nearby structures and vegetation, and feed line / matching loss. Nevertheless, it should usually fall in the range of 10 to 13 for good wire dipoles, and if you measure a half wave dipole antenna system to have Q significantly outside that range, it is probably significantly less efficient than it should be. Continue reading Assessing the Q of a half wave dipole antenna system
This article documents a field strength survey of an M40-1 short helical vertical on 40m.
This test is more a feasibility study of the experimental method and apparatus than an absolute measure of the antenna.
The antenna under test is described at AUT – MobileOne M40-1 40m helical.
Field strength was measured using a small square untuned loop and VK3AQZ RF power meter (RFPM1), and data was captured using A prototype data logger for RFPM1.
VK3AQZ RF power meter (RFPM1) described my build and calibration of the RFPM1.
Above is the RFPM1, shown with two probes, but only one probe is required for this procedure, the other is disconnected. The RFPM1 directly reads input power in dBm.
The loop antenna used was described at (Duffy 2007). It is a small square loop (600mm sides) fed in one corner with a 1:1 voltage balun. Continue reading Field strength survey of an M40-1 short helical vertical on 40m
At Improved cooling for the MFJ-949E I described a modification to the ATU to improve its cooling using a fan and run on timer.
The run on timer described was based on a Chinese STC15F104E DIP8 8051 like microcontroller.
Because the programming tools for the STC chips work so poorly, and the lack of documentation of their protocol, there is no simple way to update only the calibration data in EEPROM. I have ported the algorithm to an ATTINY25 which doesn’t cost a lot more but had a much better development environment and a range of tools to allow EEPROM update without overwriting the FLASH image, and as well it will run my bootloader, ATB.
This article describes a generic run on timer based on an Atmel AVR chip, a ATTINY25 though the code will also run in ATTINY45 and ATTINY85.
The circuit is very simple, the DC output from the forward power detector is connected to the input pin which turns the BC548C transistor on at input voltage greater than about 0.7V. The high value of base resistor ensures very light loading of the forward power detector.
Continue reading A generic run on timer using an ATTINY25
Sixth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.
This article documents measurement of impedance.
The antenna system is a G5RV with tuned feeders (9m of home made 450Ω open wire). The tuned feeders terminate on the balun described in this series, and it is located on the outside of the antenna feed entrance panel shown above. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #6
Fifth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.
Installation / testing
The balun packaged in a non-conductive housing was designed to have minimal stray capacitance to ground to minimise common mode current with asymmetric loads.
Above, the balun is attached to the exterior side of the antenna feed entrance panel using a male to male N adapter, done up very tight. The feed line connections are liberally coated with marine grease to prevent ingress of water and oxygen, a measure to reduce corrosion. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5
At A look at internal losses in a typical ATU I demonstrated that it is quite easy to raise the temperature of the coil in the MFJ-949E to an unsafe level, even with quite modest power.
The most heat sensitive component in this ATU is the coil, specifically the coil supports which are probably polystyrene, and the glass transition temperature of polystyrene is around 100°.
This article documents modification of my MFJ-949E to reduce the risk of damage under some operating conditions. Continue reading Improved cooling for the MFJ-949E
Small transmitting loops (STL) are very popular with hams, and a fashion is developing for N turn loops. This article lays out some thoughts on a 2 turn STL.
Firstly, to the meaning of “small transmitting loop’. There are a range of definitions used, and they mostly centre around the concept of a size sufficiently small that current is approximately uniform. The issue is about the meaning of sufficiently. Accuracy of estimation of radiation resistance of small transmitting loops sets out a rationale for a single turn loop for criteria that perimeter<λ/10.
This article will compare NEC-4.2 models of loops with the following key parameters:
- 1m diameter (the loop perimeter is 0.07λ);
- 20mm copper conductor;
- frequency is nominally 7.1MHz;
- 16 segments per turn
- when not specified as in free space, the loop centre is 1m above ‘average’ ground (0.005,13);
- the loop is directly fed in the middle, opposite to the tuning capacitor position, cap down;
- pitch is 0.15m.
The model is sensitive to all these parameters. Continue reading Some thoughts on a two turn small transmitting loop
The so-called End fed Zepp (EFZ) is often cited as the basis for many more recent antenna designs, and is leveraged to provide and explanation… though few hams understand how the EFZ actually works.
End fed Zepp
Above is a diagram from the ARRL Antenna Handbook (Silver 2011).
Though a short conductor is shown to the right of the right hand feed line wire, the length is not specified or discussed in the accompanying text. It is popularly held that this is a “counterpoise” that provides a path for current equal to that flowing left into the main horizontal wire.
Let us explore the EFZ using NEC. The models are a reflection left to right of the above diagram, ie the feed is on the left hand end. Continue reading End fed Zepp
David, VK3IL posted EFHW matching unit in which he describes a ferrite cored transformer matching unit that is of a common / popular style.
Above is David’s pic of his implementation. It is a FT140-43 toroid with 3 and 24t windings and note the 150pF capacitor in shunt with the coax connector.
The popular belief is that these are a broadband impedance transformer with impedance ratio equal to the square of the turns ratio, 64 in this case and therefore a broad band match from 3200Ω to 50Ω.
To his credit, David took some measurements of several different variations and reported them in his article.
Above are David’s measurements of the subject transformer.
Lets explore the matching detail for the case of a 3.3kΩ load at 22MHz, and using the 150pF shunt cap.
Superficially, you might convince yourself that this is explained by the turns squared story, but the 150pF doesn’t reconcile with that story, nor does the variation with frequency, eg the rapid change above 22MHz. Continue reading End fed matching – analysis of VK3IL’s measurements
AT Measuring balun common mode impedance – #1 I gave an example of the use of a Rigexpert AA-600 to measure the common mode impedance of a current balun.
Above is a plot from that article. I cannot be sure what version of Antscope was used to create the graph, but it was no later than v4.2.57, as one of the ‘improvements’ of v4.2.62 and v4.2.63 was to reduce zooming of the Z scales to a maximum of 600Ω. Continue reading Rigexpert’s Antscope takes a step backwards