Voltage Standing Wave Ratio is the ratio of the voltage maximum (antinode) to the adjacent voltage minimum (node) on a transmission line. (This assumes a fully developed standing wave, that the maximum and minimum are not forced by end of line). Continue reading Expression of VSWR as a simple decimal real number
Another of those threads has broken out on eHam illustrating that lots of hams do not understand the complex nature of impedance and cannot see the consequences of the formula to calculate VSWR from load impedance and transmission line characteristic impedance.
Most methods of measuring VSWR are indirect, and they are based on an assumed Zo which is purely real (ie Xo=0Ω), and we speak loosely of that as the VSWR even though the standing wave that might exist on a practical transmission line is a little different as a consequence of that assumption being a little bit in error. Continue reading VSWR=1 and X≠0
This article describes an Aerial Under Test (AUT) that features in some of my experiments and write ups and is subject of some current experiments. It is a MobileOne M40-1 helically loaded vertical for 40m installed in the car roof. It is in the style of the popular US antenna, the Hamstick, but this is a little longer and the results are not directly applicable.
I hasten to add that this configuration is not suited to travelling, it is just a rather ideal mounting of a helically loaded whip without the questions that arise from the effects of roof racks, bumper mounts etc.
The M40-1 is fitted in the centre of the station wagon roof, the roof is 1.5m above ground and the antenna is 1.5m long including a 200mm unloaded tip (tip of the antenna is highlighted with a pink dot). (The setting is not the test site.) Continue reading AUT – MobileOne M40-1 40m helical
This article describes a remote ON/OFF switch which uses an RC receiver and adapter chip to convert the RC PWM signal into an ON/OFF output. (Suitable RC transmitters are on hand.)
The immediate application is for remote ON/OFF PTT or KEY of a transmitter for field strength testing at various locations.
Remote control hobbies have long used a multi channel digital proportional protocol for control of planes etc. The simplest multi channel receiver has an independent PWM output for each servo.
The PWM signal is a 1000-2000µs pulse with a repetition rate from about 50Hz up to 500Hz or so, the duration of the pulse conveys the information.
The converter chip is a ATTiny25 MCU with firmware that monitors the PWM stream and provides ON/OFF and OFF/ON output pins. For the immediate application, the ON/OFF (or non inverted) output drives a 2N7000 FET with ‘open collector’ output suited to the PTT and KEY lines of most modern transceivers.
The firmware ignores PWM signals with duration outside the range 900µs to 2100µs, and switches ON at 1600µs, and OFF at 1400µs to provide some hysteresis. If PWM input is lost for 125ms, the output will fail safe OFF.
Above is the schematic. The 2N7000 is good for 60V, can handle up to 100mA without a heat sink, and had a body diode to absorb transients if the load is a relay. Continue reading RC PWM – ON/OFF switch
The WIA announced on 19/11/2015:
The World Radiocommunication Conference (WRC-15) in Geneva has agreed on a secondary allocation of 5351.5-5366.5 kHz for the Amateur Service, with regional power limits of 15 watts to 25 Watts measured in effective isotropic radiated power (EIRP).
This prospect has lots of hams excited, and some wringing their hands over the power limit expressed in the release.
Compliance with this power limit (1W EIRP in this region) may well challenge most modern hams (though EIRP power limits are not new to the Amateur LCD) and perhaps if that is to be the limit, a set of simple compliance guidelines be endorsed by ACMA to assist compliance. Continue reading Proposed 60m amateur band power limit
We see more and more reference to “QRP antennas” online these days, and it begs the question, what makes an antenna more or less suited to QRP.
To a novice, the obvious possibilities for a low power antenna system are that they are:
- highly efficient to offset the lower power; and/or
- unable to withstand higher power.
Ted Hart inspired interest in loops for transmitting applications with his article “Small, high efficiency loop antennas” (Hart 1986).
He included a table of recommended designs, the following is an extract of the table rows relating to an octagonal loop with perimeter=20′ (6.1m). The tube specified was 3/4 copper pipe which has an OD of 22mm. Continue reading Reconciling W5QJR’s loop formulas
(Boswell et al 2005) discussed a small transmitting loop (STL) and offered predictions and measurements of performance.
Boswell’s loop is 1m diameter of 22mm diameter copper tube.
This article is a reconciliation of Calculate efficiency of vertically polarised antenna from far field strength with Boswell’s predictions, measurements and efficiency calculations.
Above, Fig 6 from Boswell shows his prediction of the field strength of a 100% efficient loop at several distances, and measured field strength. He calculated efficiency from the difference between predicted lossless and measured. Continue reading Reconciliation of field strength to efficiency calculator with Boswell’s loop measurements
(Belrose 1998) described a mobile antenna system which, to his credit, the author validated its performance by making a series of field strength measurements and calculating radiation efficiency.
It appears that Belrose has assumed the antenna is omni directional for ground wave, though he shows that for higher angle space wave it is not omni directional.
Belrose’s measurement and calculation
Above, Belrose gives a set of measurements of field strength at different distances, and a curve fit from which he takes a value of 101.42dBµV/m at 100m as the basis for his efficiency calculation. Continue reading Belrose field strength measurements of his 80m mobile whip
There are a host of design tools for Small Transmitting Loops, spreadsheets, online calculators and conventional applications you download and run on your PC.
Almost all ignore capacitor loss… and I say almost so that I am not wrong, I have never seen one of these tools that does include capacitor loss.
NEC study of Small Transmitting Loop Q vs frequency contained a graph of the elements of feed point resistance from and NEC-4.2 model for a small loop. Key parameters are:
- Octagonal loop of 20mm copper with area equal to that of a 1m diameter circle, loop perimeter=0.104λ at 10MHz;
- centre height=2m;
- ground=0.007/17; and
This analysis only extends up to 10MHz, because for perimeter>λ/10, the formulas used by most of these simple calculators are in error for other reasons.
Above, the four elements on log scale. Continue reading On ignoring capacitor losses in Small Transmitting Loops