Analysis of a practical non resonant dipole scenario on 160m

Non-resonant dipole with two wire feed line and T match ATU discussed some of the issues with the common multi band configuration with emphasis on the lower bands where ATU losses can be sufficient to cause internal damage.

This article explores a scenario that came up in discussion with another ham.

His scenario is a moderately long dipole (just under λ/2) fed with a moderately long length of nominal 450Ω windowed ladder line… I am being obscure, but I don’t want to dwell on the details from that angle. This is not a case of someone loading up the roof rain gutters, or window frames, it is a serious antenna.

The chap is using a MFJ-941E ATU, which appears to use the same componentry as the MFJ-949E with which I am very familiar, the 941E appears to be a version of the 949E without internal dummy load.

So, he was able to get a good match easily using the internal balun (4:1 voltage balun), and from the settings we can estimate the matching and particularly the losses.

A model

I have separately come to a view that the Q of the 949E inductor at ‘A’ switch position is about 170 @ 1.8MHz. I also measured the magnetising impedance of the balun, it was 2.8+j350Ω @ 1.8MHz.

Above is a model capturing: Continue reading Analysis of a practical non resonant dipole scenario on 160m

Non-resonant dipole with two wire feed line and T match ATU

This article discusses a very popular HF antenna with hams, the non resonant dipole centre fed with two wire line. Some ‘experts’ call this a doublet, but their distinction is not captured in the IEEE Standard for Definitions of Terms for Antennas which considers doublet and dipole as equivalent.

Whilst these antennas can work well, remembering that all antennas “work” and “any antenna is better than no antenna”, is a deeper understanding useful?

As a basis for discussion, an NEC-4.2 model of a 60m centre fed dipole at 15m over ‘average ground’ (σ=0.005, εr=13) and 30m of lossless 400Ω VF=1 feedline was built. It is almost λ/2 at 160m, so might appeal as potentially useful down to 160m.

Above is the geometry of the example antenna.

 

Above is a Smith chart plot of Zin to the feed line from 1 to 30MHz, the cursor is at 1.9MHz, and this impedance 14.4-j181Ω will be used in the following discussion. Continue reading Non-resonant dipole with two wire feed line and T match ATU

K3EUI’s MyAntennas EFHW on 80m

Barry, K3EUI, posted some interesting measurements of his MyAntennas EFHW which he described with some useful detail:

I have been experimenting with a “MYANTENNA” 130 foot long “end-fed” with the 49:1 UNUN*

I replaced their 130 ft antenna wire with a heavier gauge #12 stranded insulated wire (I had a few hundred feet).*

This is classified as a ONE-HALF wavelength antenna on 80m, hence the need for the 49:1 UNUN to transform 50 ohm (coax) to a few thousand ohms*

It has resonances on the other ham bands (harmonically related) but I wanted it mostly for 80m.

One leg runs about 60 ft horizontally to a tree and then the next 70 ft makes a 90 degree bend (to fit into my yard) still horizontal.*

At this time I removed a 15 ft “counterpoise” wire on the GND side of the UNUN.

I will try it later this week (after the snow) as a “sloper” or an Inverted V up to a tall fir tree.

It is only about 20 ft above ground now (with 4 inches snow) for NVIS prop, and fed with 70 ft of RG213 coax (50 ohm) with a RF choke on the coax feed line 10 ft from UNUN (the counterpoise?) and another RF choke just as the coax enters the shack.

His VSWR curve is interesting, a minimum at source end of about 1.32 @ 3.66MHz as built and measured.

Minimum VSWR is about 1.32 @ 3.66MHz. Continue reading K3EUI’s MyAntennas EFHW on 80m

The black art of common mode current and two wire transmission lines

One of the very popular topics on ham social media is common mode current, and it seems whilst opinions are presented as fact, there is little sound science in evidence.

In a two wire transmission line, we can get good insight into the state of current balance by measuring three currents at a point along a transmission line:

  • I1 in one conductor;
  • I2 in the other conductor; and
  • I12 being the sum of the currents.

These can be measured using an RF current probe, essentially a current transformer for RF, and in the case of I12, it is measured by placing BOTH conductors through the centre of the current transformer so the flux is due to I1+I2 (not simply |I1|+|I2|). There are other ways to obtain I12, but in concept they work the same as passing both conductors through one current transformer. Continue reading The black art of common mode current and two wire transmission lines

Youtuber on “The myth of SWR”

A Youtuber recently published some enlightenment entitled “The myth of SWR”.

He is obviously a disciple of the late Walt Maxwell and his re-re-reflection explanation:

Here’s a sample of what’s coming which may make your head explode. This is from antenna engineer Walt Maxwell, W2DU.

Now this video has been published just two weeks, it has:

  • 13,693 views in two weeks;
  • 549 likes;
  • 228 comments mostly positive, but a few voices of reason.

Continue reading Youtuber on “The myth of SWR”

Youtube – measuring velocity factor of coax cable

I keep being offered Youtube videos showing how to measure velocity factor of coaxial line.

I did indulge one this afternoon. The author explains that measuring s21 phase is the basis of his measurement.

The DUT for the demonstration is 3.76m of coax, no mention of where it was measured from and to.

No mention of the calibration details, so we might assume that a short jumper was used to connect Port 1 to Port 2 for the through test, perhaps the very one shown in the pic below.

Above is the test jig, one end of the coax (UHF plug) attaches to a UHF-F to SMA-M adapter which is attached to the VNA. The other end of the coax appears to connect via a UHF-M to a UHF-F to SMA-F adapter, and the 100mm long jumper cable. Continue reading Youtube – measuring velocity factor of coax cable

A desk study of a matching scheme for a short base loaded Marconi on 137kHz

A correspondent recommended a simple configuration of a base loaded shortened Marconi for 137kHz, referencing an online posting by another ham.

I was  assured that this configuration is simple, very effective and very popular. It has been used for a very long time, so it must be good.

Well, let’s do an analysis.

The recommended antenna

The online poster’s equivalent circuit of his 137kHz base loaded vertical. The resonant frequency of this circuit is actually 136.979kHz, let’s assume the inductance is correct and that C is C=99.969pf and the circuit is resonant at exactly 137kHz. Continue reading A desk study of a matching scheme for a short base loaded Marconi on 137kHz

A desk study of a matching scheme for a cap hat loaded Marconi on 137kHz

Reworked for average ground type (σ=0.005, εr=3) …

A common scheme for narrow band match of an end fed high Z antenna discussed discussed the kind of matching network in the following figure.

A common variant shows no capacitor… but for most loads, the capacitance is essential to its operation, even if it is incidental to the inductor or as often the case, supplied by the mounting arrangement of a vertical radiator tube to the mast. In any event, and adjustable capacitor may be a practical addition to help with matching under varying environmental factors.

This article is an expose on technique rather that a recommended antenna design. Continue reading A desk study of a matching scheme for a cap hat loaded Marconi on 137kHz

Is |Z| a really useful metric for optimising antenna systems?

One often sees some misconceptions about the relationship between VSWR and impedance. The maths of the relation is explained at Telegrapher’s Equation. The relationship is not trivial and will challenge readers who do not understand complex numbers and exponentials.

Even if you do not have the requisite maths, the following examples may dispel some wooly thinking.

By example

|Z|=50

I have created a SimNEC model to simulate a load Z of specified |Z|, and to sweep the phase of Z from -90 to +90°, and to display VSWR50.

Above is the result where |Z|=50 and for phase of Z from -90 to +90°. Continue reading Is |Z| a really useful metric for optimising antenna systems?

Noise and Quasi Peak

The article Can we find the noise power captured by a 50Ω antenna and ambient noise figure using a SDR or spectrum analyser? gave a brief explanation of Quasi Peak (QP) as applied to noise measurement.

I say “noise measurement” loosely, because since noise is a random phenomena, what we do is to sample the phenomena over some period of time, and if we were to sample it again, whilst it might seem to be the ‘same’ noise, the random events  are new, they are a new set of samples, and summary of them is new. This contributes to the large variance observed in noise ‘measurement’.

QP is commonly used to express the magnitude of noise in the context of emissions and interference potential. QP derives from a very old CCIR recommendation describing the response of a noise measurement instrument. The recommendation included a specified meter response to short tone bursts. The QP detector has a slowed rise time, and slower decay time, so it under captures isolated very fast impulses, but accumulates high repetition impulses. The QP response reads higher than an average power detector, and lower than a peak power detector. My experience is that QP is about 4-5dB higher than average power on white noise, and typically around 6-7dB higher on off air noise with little impulse noise, but high impulse noise can result in a much higher QP reading. For more information, see (ITU-R. 1986).

Most EMC receivers and Spectrum Analysers with EMC feature have a QP ‘detector’. Communications receiver S meters often have a nearly QP response.

Above is a plot of sampled noise and QP calculation (the QP ‘detector’) made during development of FSM (Field Strength Meter) software. Observe that impulses have effect but the QP is closer to average power than repetitive peaks, much closer than infrequent impulses.

Note that Peak/Avg power for a sine wave is 3dB, QP/AVG power for a sine wave is almost 3dB. Continue reading Noise and Quasi Peak