I saw a question posed online about the merits of a proposed antenna system which used a hybrid feed arrangment as 15′ (sic) of the feed line needed to be buried.
Above is the poster’s diagram, and his posting lacked some important details so let’s make some assumptions. Lets assume the antenna is at 150′ in height above average ground, and since the dipole is long enough to be usable on 160m, let’s study it at 1.85MHz.
Input impedance of the dipole under that scenario is around 45-j400Ω.
Let’s consider two options:
a tuned feeder option (ie open wire line all the way to the ATU); and
We often learn more from failures than successes, this exercise is one of those opportunities.
An online poster tried to validate his newly purchased MFJ-918 by measuring Insertion VSWR.
That is done preferably by measuring a good termination (dummy load) to validate that it has a very low VSWR, then inserting the Device Under Test (DUT) and measuring the VSWR as a result of insertion of the DUT.
The poster did not mention measurement of the dummy load alone, and it is a type that warrants validation.
A ham in the need of help recently asked for advice on eHam about the use of Wireman Ladder Line and the like.
After a fairly well considered, detailed and lengthy answer from on online expert, another online expert stepped in to confuse the matter with conflicting advice:
Wire resistance (loss due to current ) is not a factor with higher voltages typically seen in high impedance antenna feed applications. Attenuation loss is a factor depending on dielectric properties in VHF and UHF frequencies. Their is little skin effect below 50 MHz in wire antennas and feeds.
This comes down to line strength. I would go with the solid Copperweld for HF antenna work.
The loop appears to be made from 7/8″ copper tube, and is 7′ in diameter. He estimates its efficiency to be 66% and initially reports I’ve got it less than 4 feet above ground yet it tunes flat to 1.1>1 with roughly 10kHz bandwidth.. Curiously, 10kHz is the result calculated by AA5TB’s spreadsheet, though I have written elsewhere it is deeply flawed (Small transmitting loop calculators – a comparison).
Let us assume that these figures are correctly reported, and that the unqualified bandwidth means the half power bandwidth of a matched loop.
We can estimate the efficiency of a Small Transmitting Loop (STL) in free space.
Before getting excited about the results, let us question the validity of the model. There are three important factors that question the validity of the model:
The IC-7300 is a transceiver where all heterodyning oscillators are derived from a single master oscillator.
This type of radio makes for very easy checking and calibration of frequency accuracy.
The video below demonstrates the technique.
The video used a local GPS disciplined source at 50.1MHz. The frequency was chosen to provide the greater resolution in setting the oscillator, though setting it to within 1 part in 50,000,000 or 0.02ppm is better than the stability of the oscillator (specification is 0.5ppm or 5Hz at 10MHz).
Any accurate known reference can be used, it could be WWV or the like, or even a MW broadcast station, though an accurate signal at 10MHz or higher is better.
The technique can be applied to the much older IC-7000, and many transceivers released since then, of various brands. The important thing is that ALL oscillators are derived from a single master oscillator.
I recently purchased a Surecom SW-102 VSWR meter. It looked a little like a supercharged RedDot copy.
Above the Surecom SW-102 VSWR meter with backlight and photographed under normal interior lighting. The display lacks contrast, and overall is difficult to read due to size of text, fonts used, and lack of contrast. (The pic is taken with a screen protector installed, but the evaluation is based on the bare meter with original protective film removed as it degraded readability.) Continue reading Surecom SW-102 VSWR meter review
A ham in search of good advice asked in QRZ forums dB, dBi, and dBd: I have a loose grasp of these terms and their relationships. Is there any way to rule of thumb the gain of any antenna over a dipole over earth?