DXE sell a nominal 300Ω ladder line, DX Engineering 300-ohm Ladder Line DXE-LL300-1C, and to their credit they give measured matched line loss (MLL) figures.
They make the common ham mistake of writing loss figures as -ve dB where in fact by definition they are +ve (MLL=10*log(Pin/Pout)).
The line is described as 19 strand #18 (1mm) CCS and the line has velocity factor (vf) 0.88 and Zo of 272Ω.
Let us calculate using TWLLC the loss at 2MHz of a similar line but using pure solid copper conductor with same conductor diameter, vf and Zo. We will assume dielectric loss is negligible at 2MHz Continue reading Loss of ladder line: copper vs CCS (DXE-LL300-1C)
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
- the hybrid feed arrangement shown.
Let’s get serious and use home made open wire line made from 2mm diameter copper spaced 150mm. The very popular 19strand windowed ladder lines using CCS do not have sufficient copper to give copper like performance, the single core CCS is marginal. Continue reading Loss in open wire + coax hybrid feed arrangements
An Insertion VSWR test gone wrong.
We often learn more from failures than successes, this exercise is one of those opportunities.
A 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.
Above is the poster’s test setup, his Rigexpert AA-170 is connected to the balun’s input jack using a M-M adapter. The output wires on the balun form a rough circle of about 550mm perimeter by eye. Continue reading Exploiting your antenna analyser #27
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.
Let’s examine the above quote. Continue reading Loss of windowed ladder line at MF/HF/VHF
Messi & Paoloni Ultraflex 7 coax cable is being marketed as similar to RG-213 in performance, but 7mm overall (against 10mm).
There is lots of comment by online experts questioning the claims, and critical of things like the braid coverage, copper foil etc, but without real evidence that it does not live up to specification.
Accepting the specifications for Ultraflex 7 and Belden 8267 (B8267, RG-213) for a moment, how do they compare.
Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.
Above is a comparison of the cables. Continue reading Messi & Paoloni Ultraflex 7 coax cable
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
The term “common mode current” applied to coaxial transmission lines is bandied about with abandon these days in online fora, awareness of its existence has increased if not understanding.
A simplistic analysis is that in TEM mode, ONLY differential current is supported inside a coaxial line, ie that at any point the current on the outer surface of the inner conductor is exactly equal to a current in the opposite direction on the inner surface of the outer conductor.
But, lets look at the wider context of the meaning of common mode current when a uniform coaxial line is connected to an antenna system. Whilst an antenna might have an obvious two terminal connection to the feed line, in the presence of ground, the current in those two terminals are not necessarily equal and opposite. Continue reading Does common mode current flow inside coax?
Find coax cable velocity factor using a very basic analyser
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure OSL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.
The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.
It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.
The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26
A convenient list of ‘Exploiting your antenna analyser’ and short subject sub-titles, a table of contents for the series as it grows.
Exploiting your antenna analyser #26 Find coax cable velocity factor using a very basic analyser
Exploiting your antenna analyser #25 Find coax cable velocity factor using an antenna analyser without using OSL calibration
Exploiting your antenna analyser #24 Find coax cable velocity factor using an antenna analyser with OSL calibration
Exploiting your antenna analyser #23 Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser Continue reading Exploiting your antenna analyser – contents
At Rigexpert Antscope v4.3.1 released I commented on a new release of Antscope.
Correspondents have asked where I obtained v4.3.1.
Well, it seems the Rigexpert website is broken again, the URL to list the Antscope downloads produces garbage. Nevertheless, you can get a directory listing at https://www.rigexpert.com/files/antscope/ and yes, you will note that v4.3.1 is not listed… so it seems to have been either pulled due to defects or it is just a consequence of the web site problems.
Little loss, I use v4.2.57 on Rigexpert’s advice as it has better scales for impedance plots… and v4.2.57 is still published (at the time or writing) https://www.rigexpert.com/files/antscope/antscope040257.zip .