Transmission lines – Page 24

Exploiting your antenna analyser #17

Optimising a G5RV with hybrid feed

(Varney 1958) described his G5RV antenna in two forms, one with tuned feeders, and the more popular form with hybrid feed consisting of a so-called matching section of open wire line and then an arbitrary length of lower Zo coax or twin to the transmitter.

(Duffy 2005) showed that the hybrid feed configuration is susceptible to high losses in the low Zo line as it is often longish, is relatively high loss line and operates with standing waves. Varney did offer two options for the low Zo line: any length 72Ω twin or coax. Continue reading Exploiting your antenna analyser #17

Last update: 9th May, 2016, 10:37 AM

Exploiting your antenna analyser #16

Measure inductor using SOL calibration

At Measuring balun common mode impedance I showed a method of backing out the effects of a text fixture using the “subtract cable” facility of Antscope software with the Rigexpert AA-600.

Some analysers (including the AA-600) support SOL calibration of the instrument itself, and some support SOL calibration using the client software (Antscope in this case). This article demonstrates use of Antscope with SOL calibration to measure a small RF inductor which has similar characteristic to good Guanella 1:1 HF baluns.

The text fixture used for this demonstration is constructed on a SMA(F) PCB connector using some machined pin connector strip, and SMA(M)-SMA(M) and N(M)-SMA(F) adapters to connect to the AA-600.

Above is a pic of the test inductor in the test fixture with adapters. The test inductor 6 turns of 0.5mm PVC insulated wire wound on a BN43-202 binocular balun core. Continue reading Exploiting your antenna analyser #16

Last update: 16th October, 2020, 6:59 AM

Improved cooling for the ATR-30

In Improved cooling for the MFJ-949E I described a solution to a problem of demonstrated overheating of the ATU at rated power, indeed at a lot less than rated power.

Though I have never measured the ATR-30 temperature rise, and am probably unlikely to stress the 3kW rated ATU with a 100W transmitter, I have performed a similar cooling modification to the ATR-30.

Continue reading Improved cooling for the ATR-30

Last update: 29th April, 2016, 7:04 AM

Exploiting your antenna analyser #15

Measure MLL using the half ReturnLoss method – a spot test with a hand held analyser

At Exploiting your antenna analyser #14 I gave an explanation of the method of approximating MLL of a line section by taking the average half Return Loss with o/c and s/c terminations.

This article demonstrates the technique using the Rigexpert AA-600 analyser in hand held mode.

The task is to assess whether a section of RG58A/U coax has MLL at 3.5MHz similar to specification or not.

The specification loss of 10.13m of RG58A/U has MLL=0.29dB.

Above, the first test with an o/c termination. Return Loss is 0.4dB. Continue reading Exploiting your antenna analyser #15

Last update: 25th April, 2016, 10:16 AM

A generic run on timer using an ATTINY25

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

Last update: 14th April, 2016, 7:32 AM

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #6

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.

Impedance measurement

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

Last update: 13th February, 2017, 4:34 AM

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5

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

Last update: 13th February, 2017, 7:46 AM

Improved cooling for the MFJ-949E

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 melting 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

Last update: 25th April, 2020, 6:55 AM

End fed Zepp

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

Last update: 14th March, 2016, 7:08 PM

Thinking about SOTA, EFHW and EMR safety

There seems to have been a revival in use of the so-called End Fed Half Wave antenna.

The prospect that a small radio such as the FT817, a magic match box and 10m of wire makes a good 20m field station appeals to many a SOTA enthusiast.

Let us model a scenario with a FT817 powered by internal battery and sitting on an insulating platform (eg a pack) 0.3m above natural ground, a 10m wire strung up into a tree at a 45° angle, and a 1m long mic cord stretched up at 45° in the other direction. The is the popular so-called ‘no counterpoise’ configuration.

A simplified model of just the current paths without regard to the bulk of the radio, or the effect of the helix of the mic cord illustrates an approximate current distribution. The model uses 1W RF input to the antenna over ‘average ground’ (σ=0.005, εr=13).

Above is a plot of the current distribution. Current is a minimum at the open ends, a boundary condition for the problem, and maximum in the middle of the half wave. We expect H field to be greatest near the current maximum, and E field to be greatest near the current minima. Continue reading Thinking about SOTA, EFHW and EMR safety

Last update: 27th April, 2016, 6:00 PM