A recent online posting provides content for learning. K3EUI posted a NanoVNA-Saver screenshot of his antenna described as:
Set out a horizontal loop wire antenna for possible NVIS paths
Wire is about 140 ft length with an outside CLC tuner, fed with 50 ft RG213. …Here were the Nano VNA graphs of this new loop antenna, measured from inside the shack (50 ft RG213)…
Can we learn something from this? Continue reading Antenna assessment using NanoVNA – learning from a user example
A ham consulting the experts on QRZ asked:
On 30 meters, my SWR reads 3:1 to my antenna (an EndFed 53 feet long wire up about 25 feet). Reading a chart I have, I see that at 80 watts output, my reflected power should be 20 watts. I verified this by looking at my Diamond SX-200 Meter which also indicates the reflected power is 20 watts. My questions are these: does the 20 watts reduce my 80 watts output to 60 watts at the antenna? I have a choke on my feed line in my shack (near my transceiver) & the SX-200 Meter is between the choke & transceiver….
The OP later explained that the transceiver is a IC-7300 and it appears that the internal tuner is in use above… so let’s proceed on that basis.
Analysing the OP’s report, his SX-200 indicates VSWR=3 Pf=80, therefore Pr=20, and P=60W. Note that \(P=P_f-P_r\) is valid because Zref is real, so the answer to his question about power to the antenna is 60W, he is quite correct.
He went on to ask where the 20W reflected goes to… but I will leave that to Walt Maxwell devotees to discuss energy sloshing around and re-re-re reflections… the stuff of ham lore.
As an example of what might be expected of the IC-7300 with a mismatched load, I did a series of measurements at 7MHz with a sample variably mismatched load.
Above is a plot of power output vs VSWR for a sample mismatched load. Also plotted is the measured reflected power and the calculated power output based on the ham lore \(P=P_f (1-\rho^2)\). Continue reading IC-7300 VSWR protection
I recently posted a video entitled Half wave dipole series match .
Readers may find it relevant and interesting.
A correspondent reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna referred me to (Healey 1969) and questioned my method.
I have tried several times to reconcile built and tuned antennas and NEC models with Healey and failed, leading me to think of the problem and devise a good approximation that did reconcile (for me).
This article attempts to reconcile the example given at Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna, an example where the two built antennas reconcile well with the ARRL published design article and NEC model.
The example antennas are 4 element 144MHz Yagis built around 1970. They were originally designed with a 50Ω split dipole feed, or the option of a folded dipole with 4:1 half wave coax balun. Continue reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna – comparison with Healey
Seventh 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.
The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).
Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7
Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna posted a Simsmith design tool to assist in designing a Gamma Match.
Let’s walk through an example.
Above is an example for discussing the Gamma Match. In this case, the assumed feed point impedance of a simple split dipole feed is 17+j2Ω, and the challenge is to design a practical Gamma Match to match it to 50+j0Ω.
The design tool assumes that the connections to the open circuit stub are at the feed point, ie that the gap in the gamma arm outer is at the inboard end. There are other ways to build a gamma match and the model may not suit them without tweaking. Continue reading Designing a Gamma Match – Simsmith design tool – how to
Designing a Gamma Match – confirmation of as-built antennas was based on some online calculators to provide key values to a Simsmith model of a Gamma Match.
This article provides an updated Simsmith model that incorporates the necessary calculations (ie without depending on external calculators).
Much is written about the virtue of the Gamma Match, and near as much about how they work, and the difficulty in design and implementation.
Designing a Gamma match using a Smith chart showed a design method for a simple Gamma Match using a Smith chart as the design tool.
This article visits the implementation on a pair of antennas that I built 50 years ago, and are still in use today (albeit with some small preventative maintenance once during that interval). The basic antenna is a four element Yagi for 144MHz copied from an ARRL handbook of the time, probably based on NBS 688. It was designed to deliver a split dipole feed point impedance of 50+j0Ω.
I built them using a Gamma Match, partly to get some familiarity, but mostly to implement a Gamma Match that was reliable, weatherproof and lasting… features that are alien to most implementations I had seen at that point.
Both antennas were constructed and the Gamma Match adjusted for VSWR<1.1 using a Bird 43 directional wattmeter. The dimensions of each (including the key gamma dimensions) are the same, not surprising, but a confirmation of repeatability. See Novel Gamma Match Construction for more discussion.
Above is a dimensioned drawing of the construction. Continue reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna
At Where is the best place to measure feed point VSWR I discussed location of the VSWR meter and projection of its reading to another point on a known transmission line.
One of the conclusions drawn in that article is:
Feed point VSWR can be estimated from measurements made at another place if the transmission line parameters are known. It, like all measurements, is subject to error but it may be a manageable error and indeed possibly better overall than direct measurement.
This article discusses some issues that may arise in referring measurements from one place to another (eg near transmitter to antenna feed point).
Let’s consider two categories of transmission lines in terms of characteristic impedance Z0 and propagation constant γ:
A lot of theoretical analysis uses lossless line for simple explanations, and whilst for a lot of purposes, approximation of practical line as lossless line serves well, at other times the error may be significant.
A Lossless Line has imaginary part of Z0 equal to zero and the real part of γ equal to zero.
A practical line has non-zero imaginary part of Z0 and non-zero real part of γ, and these are frequency dependent.
Under standing waves, attenuation along a practical line is not uniform, in most practical applications conductor loss/m is higher than dielectric loss so loss is higher near current maxima than near current minima.
For the purpose of this article, it is the frequency dependence of Z0, particularly the non-zero imaginary part that is significant.
A model of a load similar to a 7MHz half wave dipole fed with 10m of RG58A/U was created in Simsmith to provide a basis for discussion. Whilst the model is subject to some errors computation, it is much less than comparing two field measurements at both ends of a transmission line.
Let’s look at the ACTUAL VSWR. Actual means that if you were to observe the standing waves on the line (eg with a voltage probe), this is the VSWR you would expect to observe.
Firstly, observe that the source end VSWR (orange) is a little lower than the load end VSWR. This is by virtue of the attenuation on the line. The difference between the two can be calculated, but it is moderately complicated. Continue reading Where is the best place to measure feed point VSWR – error in Z0
This article continues on from Implementation of G5RV inverted V using high strength aluminium MIG wire documenting review after 7 years operation under a wide range of temperature, humidity and wind conditions. It adds some contemporaneous pics of some parts of the system.
Above is a view of the steel mast (2021) with the Inverted V G5RV rigged from the top of the 11m mast using a halyard through a purchase on a small gibbet to offset the antenna and feed line from the mast. There are lateral guys at 7m height, and the left hand one is non-conductive synthetic fibre rope. Atop the mast is a 2m/70cm vertical.
Above is a view of the feed point (2023) of the G5RV at 1mm AGL. The galvanised telescopic TV mast is 55 years old and starting to show signs of rust, likewise for the gibbet which was painted with cold gal back then. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire – 7 year review
This article explores the relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals.
For clarity, \({RadiationEfficiency}=\frac{FarFieldPower}{InputPower}\).
Such antennas are often advertised with a “minimum VSWR” or “VSWR at resonance” figure, but rarely show gain figures. One might wryly make the observation that that is how one might sell dummy loads rather than antennas.
Well, these things do radiate, so they are not very good dummy loads. Lets explore a theoretical example on the 40m band to inform thinking.
Above is a NEC5.2 model of a vertical on a wagon roof. Continue reading Relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals