DG7YBN - Low Noise Yagis
 

Measuring Antennas  
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Plotting Antenna VSWR right

  Last Update May 12th 2017


Minimum Setup for plotting
Where does the Far Field start for a Long Yagi?
Verifying results on a wide open flat hill top
How to set up a Measurement Yagi
How to tune a Bent Dipole Yagi




How to setup antennas for reliable VSWR or Return Loss plots

This is how I plot my 70 cm Yagis for the quick reference in my yard. For the full reference I pack everything into my car and drive to a flat hill top plateu with no houses or trees around for a couple of 100 meters.

But anyway, a configuration as shown in the sketch below will do for very most applications unless we want to define new BC numbers or similar.





Minimum Setup for plotting a 70 cm Yagis VSWR


1. Make sure not to get hit by a falling down antenna or pole (!). Loose masts and poles are a potiental source of danger. Do not set up a loose pole near high voltage lines or where people might get hurt.

2. Get the main beam lifted from touching gnd or anything else in range of approx. next 20 meters by tilting the pole. Which is in accordance with what DL6WU says about measuring VHF / UHF Yagis.

3. Let the coax drop down in wide loop. This is about not producing anything close to an odd multiple of 1/4 lambda loop against boom or pole ... which might interfere with SWR etc. It is a good idea to try different fixing position for feed coax if antenna seems to misbehave and see if that does anything to the SWR.

The old school saying: Near Field and thus reactions to environment would end at 1 lambda or so from the antenna might be true for dipoles, NOT for long Yagis though. With the test antenna builds of YU7EF designs for WiMo one could trace or detect cars or even pedestrians walking by in 30 m distance on the VNA plot line (!). Find more read on this in next chapter.








And producing a REAL Return Loss plot
by tilting a Longyagi for 144 MHz with right equipment


A GTV 2-14w built by Jörg, DL2VL and tilted by ~ 30 degrees on a crank mast in sufficient height.

This is exactly the way to achieve a proper Return Loss plot. Yes, I know .. such equipment is not accessible by everyone. But as phyiscs are that way there is no escape unless relying on well built up know how paired with experience maybe.

Image Source: Photos and build by Jörg, DL2VL











Where does the Far Field Region start for a Long Yagi?

From my article "Applied Conversion of Segmented Wires from NEC2 to 144/432 MHz Yagi Elements - Part 3", Dubus 2/2011 & Dubus Technik XI

"Most Radio Amateurs do understand the Near Field Region where the Antenna’s impedance will be influenced by the environment as 2 times the wavelength from a Dipole. Citing typical Amateur Radio Lecture Websites show the following: “The Near Filed reaches out a half to maximum one wavelength around the antenna”. That might be true for Isotropic Radiators, Dipoles and alike but certainly not for highly directive antennas. Sadly that statement has become a conventional wisdom that is falsely applied on all kinds of antennas in Amateur Radio circles. If we want to measure Long Yagis correctly we need to get a revised picture of where the Far Field Region of our actual build will begin. In order to achieve a true Return Loss measurement we must stay clear of Near Field and Fresnell Region at least for the main beam.

The Radar Theory shows an equation to calculate the end of influences i.e. start of Far Field region that delivers much larger values. Taking an example wise 6.0 m long Yagi for the 2 m Band we derive at the following figure:

Discussion: The common statement above may be useful for non-directive antennas. The Radar Equations Formula derives from Dipole Arrays were D or L is the Length of the line of Dipoles. It is valid in the Beam Direction. None of the above is absolutely fitting to a long Yagis behaviour. But this outline may show that it is important were to beam at or were not to beam at and how much of free space in front of the measurement Yagi is really needed. Clearly the 34 m for the example wise chosen 6 m Boom Long Yagi does not reflect it’s Q-Factor. But from the manifold of test measurements we did I conclude that this distance of minimum free space seems to be fitting."









Verifying results on a wide open flat hill top

For whatever real reference measurement we need an area with nothing disturbing the antennas Near Field and Fresnel-Region. The photo shows the author with a 2 m band YBN 2-5 on wooden lace as boom on his 'home hill'. Note the distance to trees in the background. Exact measurment of antennas inbetween houses and trees does only work to a limited extent on VHF.

Analyser and Laptop are hung up an on a 12 VDC to 230VAC converter for camping










How to set up a Measurement Yagi the right way?

This paragraph is about essential understanding of fundamental working prinples of the Yagi-Uda. Without you might get lost in false assumptions when tuning your homebrew Yagi antenna.

The Driven Element (DE) or Dipole is subject to influence of mounting plate or box and boom correction in case of being mounted narrow above boom. But probably most of all the connection to the feeding coax being imperfect by means of split up length to connect core and shield.

Fig. A1 shows the 'as best as we get it' condition. Shortest split ends and the right portion of BC or Box compensation applied.

Fig. A2 shows how we should deal with pigtail lengths if only we knew the factor i.e. exact compensating span width prior to testing.

Fig. A3 is very common but worst scenario. Why so? We leave the DE too long because cutting is easier done than prolonging it later on. Second the coax is connected without caring for the split length and what a few millimeter mean on the designated VHF/UHF band.

Source of Image: DG7YBN, Applied Conversion of Segmented Wires from NEC2 to 144/432 MHz Yagi Elements - Part 3, Dubus 2/2011 & Dubus Technik XI

Now, why is the above so important?

Actually it is not that important, as many a proven builds done without that care working out there show. But if you want to achieve the best and true pattern all has to be fitting. So read on please.

Below we find sketch A4 of a Yagi-Uda holding two envelope lines. The inner, black one symbolises an element set with no or not sufficent BC. Whilst the outer, green one resembles a Full BC as regular BC plus added compensation for a models high segmentation density (SBC) in case. For the very most (non special Low Noise) Yagi designs we will find the too long a starting length of the DE in addition to the long pigtails working as 'force that pulls down' the Yagis response frequency. And thus disguises the need for more BC than applied.
So that when not plotted very carefully and judged with experience the VSWR seems alright and the Yagi goes up the pole. But alas, with typically less 2 or 3 mm on all element lengths. With a spoiled pattern as far as max. gain is concerned. It will drop by approximately 0.2 dB and, hurting far more in EME and contest use, F/B and F/R are down by several dB in most cases.

Source of Image: DG7YBN, Applied Conversion of Segmented Wires from NEC2 to 144/432 MHz Yagi Elements - Part 3, Dubus 2/2011 & Dubus Technik XI

How to get a grip on finding the true length for a DE?

It is recommended to produce adjustable end tips. Which can be done by cutting threads into the tubes used and fit screws with arretation nuts into these.

• A tube 6 x 1 fits a metric thread 5 x 0.8 mm ISO for regular screws M5
• A tube 8 x 1 fits a metric thread 6 x 1 mm ISO for regular screws M6
• A tube 10 x 1 fits a thread 8 x 1.25 mm ISO when the ends are compressed a little

Source of Image: DG7YBN, SM5BSZ’s BC.exe modified for frequency scaling and tested on 432 MHz, Dubus 4/2012 & Dubus Technik XIV

• And finally: another masterpiece out of DL2VL's workshop holding a fine thread with a pitch of 0.5 mm. Thread is 8 x 0.5 mm for which the ideal tapping drill hole should be Ø 7.5 mm. Which he passed by in compressing the slotted end in the turning lathes collet and cutting the thread in there.

Source of Photo & make: Jörg, DL2VL








Detailed Tuning Instructions for Bent Dipole Yagi-Uda Antennas

Antenna feed point impedance Z = R + jX with R as “Real Part” and jX as the “Imaginary Part”. While R is the ohmical feed point resistance the jX is of capacitive or inductive nature. For a bent dipole arrangement this simply put means R is transformation ratio to 50 ohms inside the Yagi to antenna terminal. While jX accounts for the length of the dipole in context to the Yagi-Uda structure being too long or short if not zero.

Using a VSWR meter we cannot monitor changes on the individual parts of Z. We do not see R alone nor jX. Thus we access the superposition of both as Z only. And here comes the challenge: As for example Z = 41 + j 29 ohms makes 50 ohms as well as 50 + j 0 ohms do.

When bending and varying the length of the dipole we might be lured into wrong direction by the sum R + jX when tuning the antenna. Which would mean to derive non suitable bending angles which are compensated by also non fitting length of the dipole. The resulting complete band VSWR plot would not resemble the engineered one. Hence I advise to work towards the best VSWR while bending and modifying the dipoles length in iteration steps as follows:


1. Set up dipole pre bent to sketch
2. Measure the VSWR at designated frequency
3. Bent 1/2 inch back (adjust with a tape measure noting distance tips of dipole to reflector)
4. Measure VSWR (better / worse?), note that for direction
5. Add small bit of metal (screw, something) into ends of dipole, extending length by 3..5 mm
6. Measure (better / worse?), note that for direction
7. If worse, cut away 2 mm
8. Measure VSWR
9. Better? Bend a little further into the better direction
11. Still better?
12.Otherwise try same in reverse direction
13. Repeat with / without metal bits extending dipole length
14. If worse, cut away 2 mm
15. If better, play with bending angle for a further round


Once you found direction and all you bend and cut betters the VSWR you have a clear direction and you may proceed without too much back and forth and speed it up.









73, Hartmut, DG7YBN


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