The subject question is often asked, and the usual responses are mindless recitals of Rules of Thumb (RoT).
In the light of the discussion at Feed line length affect on VSWR and The half waves of coax rule for measuring VSWR accurately, lets consider the subject question and develop a rational answer. Continue reading Where is the best place to measure feed point VSWR
This article explores the way in which VSWR varies along a feed line.
The graph above shows R and X, and VSWR along a feed line with a 100+j0Ω load at 3.6MHz. The feed line is Belden 8262 50Ω coax, manufacturing tolerances are taken to be zero, and the displacement is relative to the feed point, ie -ve distance is distance before the feed point, the feed point then is at the right hand side of the graph.
Continue reading Feed line length affect on VSWR
Readers of my article Galvanised steel wire OCF dipole have asked whether the analysis applies to an ordinary centre fed half wave dipole, and this answer is no, not exactly, but very close.
The current distribution on the OCF is very slightly different to a CF dipole, and this causes a very slight change in loss, 16.55W vs 16.68W.
Fibre rope has applications in antenna rigging, typically for halyards and tails to support the ends of wire antennas such as dipoles. The use of fibre rope for guying applications has issues due to shrinkage when wet.
Much is made of the ‘best’ fibre rope for these applications, and the almost universal answer is black Dacron (Polyester) in small sizes from 2.5mmm to 6mm diameter. The smaller sizes are usually a braided construction, commonly known as venetian cord. Claims are that the Polyester rope is strong, UV proof and rot proof. Whilst Polyester rope is resistant to mild acids and alkalis, it is damaged by strong alkalis (such as might develop when an alkaline solution dries on the rope). Continue reading Synthetic fibre rope for antenna rigging
New hams are often advised quite poorly, justified by the mantra that “any antenna is better than no antenna at all”. The more confident the advisor, the more jargon, the more unsupported, the more the advice is soaked up by eager newbies. Not just soaked up, but learned so the newbie can talk like the pro… even if they also do not understand the subject matter. Spruiking BS has always been a feature of ham radio.
Recent online discussion illustrated and recommended a 40m dipole strung on a timer fence at a height of about 1.5m to a new ham with a 10W power limit in search of good solutions from ‘experts’. Continue reading Effect of ground on HF horizontal dipole efficiency
There is little doubt that overall, hams have dumbed down as technology has raced ahead.
An interesting result of this is the appearance of new antenna analysers aimed at the dumb ham.
What is the dumb ham in this context? Continue reading The sign of reactance
AS/NZS 2272.2 sets out in Appendix E a method for calculation of near field strength of antennas, citing ANSI C95.3—1973 as authority.
This article comments on application to antennas other than those with a circular aperture. Continue reading Estimating near field strength with small antennas
An opportunity presented to measure the loss of a sample of JSC windowed ladder line. The line uses single core #18 CCS conductors and is equivalent to products sold under various proprietary names. JSC Wire & Cable is now known as Seminole Wire & Cable. Continue reading Loss of JSC ladder line at 160m
In a QST column in 2008, a correspondent asked the question
… I have the ladder line terminated to double coaxes that run about 12′ (4m) inside the house to an antenna tuner. Should this pair of coaxes be grounded at one end or both ends?
The Doctor gave a detailed diagram (above) and his advice was… Continue reading Spoiling balun action with ‘shielded twin’
Martin Ehrenfried (G8JNJ) conducted some interesting experiments on a small transmitting loop.
The loop above is 3.45m perimeter of LDF4-50A Heliax, and uses the full length of the Heliax as a stub for tuning the loop to 5MHz. Continue reading Thermograph of small transmitting loop