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DR YL/OM/SWL, many thanks for stopping by!  

On the waves, I use "Chris" for short. My logs are frequently uploaded to QRZ, LoTWeQSL, and QRZCQ.

Rig: Yaesu FT-857D, barefoot. Computer interface: minimal, homebrew. 

I like to experiment with HF digital modes and homebrew antennas:

  • VRT: For the shorter bands, I use a vertical antenna of variable length using the flat metal roof of the house as the ground plane at 6m above ground level. The vertical antenna is tunable to λ/4 resonance at all of the shorter bands from 20m to 6m (quarter-wave monopole). The foto below right shows the position of the vertical antenna highlighted in yellow. Sometimes I use a Sandpiper MV-6+3 shortened multi band vertical, instead (see further below). From May 2016 on, I also experiment with 5/8 λ verticals for the 6m and 10m bands, since my homebrew two-element LC remote antenna tuner is operational at the antenna base on the roof.
  • WBFD: Above the metal roof, I have also installed a homebrew wideband folded dipole bent into the shape of a horizontal-V for 40m (highlighted in yellow in the foto below left, the red dot marks the position of the 1:1 Guanella balun). This homebrew bent-dipole design is discussed further below. The folded dipole has replaced the earlier horizontal-V dipole for 40m with 4:1 transformer balun. That earlier homebrew bent-dipole design is also discussed further below.
  • IV:2x17m: In the backyard there is an inverted-V dipole for the 80m and 30m bands: 2 x 17m of wire hanging there in the trees. Apex approx. 8m above ground level. Lower end of the wires, approx. 3m above ground. Sorry no foto. If operated with a flimsy homebrew balun, this antenna also works on 160m (somewhat miraculously, maybe it ain't the wires that actually radiate, but the roof).
  • VHF-DIP: In May 2017, I set up a simple horizontal dipole for 144-146 MHz. Height above grond level: approx. 8m. So, now I am sometimes also fooling around on the 2m band.
WBFD: 40m folded dipole above metal roof

VRT: 20m-6m vertical monopole above metal roof

I recieved my license and call sign OE1VMC in April 2014. Since childhood, Chris is fascinated by waves: how they are created and detected. How they propagate, interact with media, interfere, and mix.

Waves carry energy and information. Credo: No thing faster. No transport more economic. 

Since 2014, I serve as president of the Radio-Amateur-Club of TU Wien with callsign OE1XTU. Current activities of the club members are documented in our qrzblog. Additional info about me is found in the web pages of the Institute of Telecommunications.

Two photos below illustrate my summer project 2014: Setting-up a homebrew 40m monoband inverted-V dipole (see left foto) above metal roof, as well as erecting a Sandpiper MV-6+3 shortened multiband vertical for 80m, 40m, 30m, 20m, 17m, 15m, 12m, 10m, and 6m (see right foto) using my metal rooftop as groundplane.

V-dipole
 

40m monoband inverted-V dipole

Sandpiper MV-6+3 multiband vertical 

 

 

 

 

 

 

 

 

 

 

 

 

To get an idea, how these antennas perform and compare on my metal roof, I have discussed the construction and geometry intensely with Gregor OE1GLC and Arpad OE1SZW. To model these antennas numerically, I use the software tool EZNEC+ v5.0 (by Roy W7EL). Detailed analysis below.

Since Spring 2015, I have spanned a second homebrew inverted-V monoband wire dipole for 80m in the backyard which is supported by trees (Apex 7.5m above ground level, legs 3m above ground level). Analysis is beginning. I keep you posted.

 

40m wideband folded dipole (up since Aug. 2016):

After some brief experiments with a horizontal loop above the metal roof, I decided to replace the 40m monoband horizontal-V dipole (see below) by a V-shaped folded dipole. The center of the dipole is approximately 1.5m above the metal roof, whereas the dipole's wires end at approx. 0.5m above the metal roof. The dipole is very near to the roof which causes significant capacitive loading. 

Generally, the folded dipole offers two advantages over the simple dipole: It is inherently more wideband than the simple dipole and features a fourfold increase in antenna impedance (comparing to the previously installed simple dipole, I expect 12.5Ω x 4 = 50Ω). Thus, a simple 1:1 current Balun (common-mode choke) can be used with the folded dipole. I hope that this design is somewhat less lossy (i.e. more efficient) than the previous simple horizontal-V dipole with the rather complicated and not so wideband 4:1 balun discussed further below. EZNEC simulations confirm that the folded dipole gives a good match to 50 Ω coax in the whole 40m band (see figure below right) which are in good agreement with the SWR measurements (see figure below left). 

EZNEC Model of the 40m folded dipole Azimuthal antenna pattern @ elevation angle 35º
Measured SWR over frequency EZNEC simulation of SWR over frequency

40m monoband horizontal-V dipole (Oct. 2014 - Jul. 2016):

This antenna is now down and the folded dipole (see above) is in its place instead. For comparison, the story is still here.

Below left, the wire model for the 40m monoband horizontal-V dipole above metal roof is shown. The center of the dipole is approximately 1.5m above the metal roof, whereas the dipole's wires end at approx. 0.5m above the metal roof. The dipole is very near to the roof which causes significant capacitive loading. The metal roof is not really plain, but slightly V-shaped with the drainage in the middle. I have approximated the roof by a square wire grid with spacing 3m (the spacing is well below λ/10). From the four corners downwards, four wires are used for modelling the lightning protection rods to the ground. The ground itself is modelled by the (lossy) MININEC model. The simulation result for the current distribution on dipole and roof is shown in magenta color. Below right, the corresponding elevation slice through the antenna's far field pattern is shown for frequency 7.1 MHz.

40m monoband horizontal-V dipole above metal roof Directivity and gain (ground model: MININEC)

The SWR simulation in EZNEC further indicates that this dipole's impedance is approx. 12.5Ω at resonance which motivates the use of a 4:1 Transformer Balun to improve the matching. The principle of the 4:1 Transformer Balun is shown in the circuit diagram below. The balun is realized by 7 quintufilar windings on a ferrite core (photo below).

Circuit diagram Photo of realized balun

Two figures below give SWR vs. frequency with a 4:1 Balun: The EZNEC simulation result is shown on the left for reference impedance Z0 = 12.5Ω. The corresponding measurement result using an MFJ-269 antenna analyzer is shown on the right.

SWR Simulation SWR Measurement
 

Simulation and measurement agree fairly well and show that this inverted-V dipole is quite narrowband in operation. The required 4:1 Balun which transforms the balanced 12.5 Ω dipole impedance to 50 Ω unbalanced coaxial line does not seem to be commercially available (any hints to the contrary are much appreciated). Therefore, a custom design for the 4:1 Balun was manufactured and measured. Many thanks to Gregor OE1GLC and OM Peter.

Is this metal roof an effective groundplane for verticals?

The metal roof is approximately square in shape (12m x 12m). Is my metal roof actually large enough to serve as a groundplane for the 40m band? Well, this depends: To get a first rough impression of the roof's effects, I have first modelled a full-size λ/4 vertical groundplane antenna for the 40m band. Below left, you find the current distribution.Below right, the corresponding elevation slice through the antenna's far field pattern is illustrated.

 

40m band full-size λ/4 vertical antenna above my not so well grounded metal roof Directivity and gain (ground model: MININEC)

The simulations indicate that the vertical antenna above the (almost square) metal roof is very omnidirectional in azimuth (this is no surprise). This dipole above the metal roof is also more or less omnidirectional in azimuth (with 3dB variation in azimuth). In contrast, their elevation characteristics are quite different.

The gain of the vertical antenna (0.84 dBi) seems to be rather poor. The vertical antenna gain depends quite critically on the earth ground model and the quality of the electrical connection from the metal roof to the ground. To illustrate this issue, I have simulated an idealized metal roof with an excessive set of wires to a perfect ground. See below left and right. In theory, we expect a gain of 5.15 dBi = 2.15 dBi + 3 dB for a λ/4 vertical antenna above a perfectly conducting infinite ground plain.

40m band full-size λ/4 vertical antenna above metal roof with idealized connection to the ground Directivity and gain (ground model: Perfect)

In conclusion: A full-size λ/4 vertical antenna which uses my metal roof as groundplane cannot meet the antenna gain of my 40m monoband inverted-V type dipole. The metal roof is a poor groundplane for wavelengths of 40m and larger. At smaller wavelengths, however, the picture changes and the metal roof becomes more and more effective.

Sandpiper MV-6+3 shortened multiband vertical:

As a start, I have setup an EZNEC wiremodel for the Sandpiper MV-6+3 above a perfect groundplane. Below left, the wiremodel geometry is illustrated. The vertical antenna parts are modelled by wires #2, #15, #16. Wire #1 is the connection to ground. The source excitation is slightly above the little dot between wires #1 and #2. All other wires (#3, #4, ..., #14) in the model are horizontal. Wires #3 - #8, and #15 carry inductive loads. These serve as individual resonators for different bands. I have modeled each inductive load in EZNEC by a series RLC circuit (with a shorted "C"). I admit that I have simply guessed the series equivalent circuit component values (Rs and Ls) for all loads. Very likely, my guesses for the losses in the inductors (the Rs values) are under-estimated.

Below right, the simulation result for the Standing Wave Ratio (SWR) is shown versus frequency from 3 to 52 MHz. It is seen that the SWR varies widely with frequency. The resonators are tuned such that the antenna shows acceptable SWR < 3 on most of the HF bands.

Wiremodel geometry Standing Wave Ratio vs. frequency
 

For my model, EZNEC calculates the characteristics summarised in the table below. On the lower bands (80m and 40m), the usable bandwidth without tuner is very small. Likely the usable bandwidth is somewhat larger than the EZNEC simulation indicates because my wiremodel is less lossy than the antenna.

 
EZNEC simulation results

 

 CUAGN, 73 es GDX de Chris, OE1VMC

 

Free counters!

 

8179962 Last modified: 2017-06-25 14:37:20, 18046 bytes

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QRZ Logbook Summary for - OE1VMC
Latest Contacts for OE1VMC at QRZ.com
dedateband mode grid Country op
R10KHL 2017-08-18 40m SSB LP13hg Russia Special Event Station M-DX-C
TF5B 2017-08-16 40m JT65 IP05 Iceland Billi Brynjolfur Jonsson
IW2IBG 2017-08-16 30m JT65 JN45 Italy CRISTIANO CROTTI
LA1MFA 2017-08-15 80m CW JP99bf Norway Kjell Berg
OM1AKD 2017-08-15 80m JT9 JN88 Slovak Republic Dezider Krizan
R2ABM 2017-08-13 40m JT9 KO85 Russia Serge V. Smagin
SV8EUL 2017-08-13 40m JT9 JN28 Greece John Gavalas
DK7OG 2017-08-13 40m JT9 JO62 Germany Hans-Andre "Andy" Ahrens
G4YKQ 2017-08-13 40m JT9 IO91 England PAUL WELFORD
S59P 2017-08-06 2M SSB JN86ao Slovenia Radioklub Radenska
OM3CQF 2017-08-06 2M SSB JN88qq Slovak Republic Draho Rojkovic
OE3XMC 2017-08-06 2M SSB JN88dc Austria Moedlinger Amateur Funk Club (MAFC)
S50C 2017-08-06 2M SSB JN76jg Slovenia RADIOCLUB DOMZALE
OE1ILW/3 2017-08-06 2M SSB Austria
SN7L 2017-08-06 2m SSB JO91qf Poland Contest Call of SP7PGK Club

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c
Grid Squared Award#13522
Granted: 2016-08-15 08:36:02   (OE1VMC)

Endorsements:
  • 5 Band Mixed
  • 15 Meters Mixed
    17 Meters Mixed
    20 Meters Mixed
    40 Meters Mixed
    80 Meters Mixed
DX World Award#3556
Granted: 2016-04-08 17:15:35   (OE1VMC)

Endorsements:
  • Mixed Digital
World Continents Award#12019
Granted: 2016-02-21 15:38:45   (OE1VMC)

Endorsements:
  • 5 Band Digital
  • 10 Meters Digital
    15 Meters Digital
    17 Meters Digital
    20 Meters Digital
    30 Meters Digital
    40 Meters Digital
    80 Meters Digital
  • 5 Band Mixed
  • 10 Meters Mixed
    15 Meters Mixed
    17 Meters Mixed
    20 Meters Mixed
    30 Meters Mixed
    40 Meters Mixed
    80 Meters Mixed
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