DG7YBN / 403 MHz / YBN RS-14w
  Last Update Nov. 29th 2014


Performance Data and Geometry
Pattern and VSWR
Download as File

YBN RS-14w   wideband Yagi for reception of  Radio Sondes

Wideband version with SWR < 1:1.5 from 400 to 406 MHz

This Yagi is designed to meet expectations for easy building but clean pattern for the necessary bandwidth for monitoring
the complete 400 - 406 MHz Radio Sonde band. It is 50 ohms direct feed, the coax cable can be connected as is.
A ferrite core added on the fed line is welcome but not essential.

This design is developed using the GTV 70-14m 432 MHz Low Noise Yagi as baseline.

Current distribution

This far the YBN RS-14w has been built by Daniel, OM1ATS and Yves, F5RBP.
Daniel states "In first test antenna has very good SWR near 403 MHz".

Yves closely followed the given geometry of the wave guiding structure and
yields SWR in place too. He even modified the designs driver cell to fit a bent
Dipole on his own.

• see here for Plot and Photos of F5RBP's Yagi

Performance Data

                       402 MHz        403 MHz       404 MHZ
Gain vs. isotr. Rad.  16.1 dBi       16.1 dBi      16.2 dBi
Gain vs. Dipole       14.0 dBD       14.0 dBD      14.1 dBD
-3 dB E-plane                        31.2 deg.
-3 dB H-plane                        33.0 deg.
F/B                  -31.2 dB       -28.9 dB      -26.9 dB
F/R                  -25.8 dB       -25.3 dB      -24.3 dB   
Impedance                              50 ohms
Mechan. Length                       3031 mm
Electr. Length                       4.07 λ

Stacking Dist. h-pol.
top-to-bottom                        1.31 m
side-by-side                         1.38 m


Runnig an SWR plot with EZNEC Auto Segmentation (11 to 8 segm.) results in 405.5 MHz.
Applying the SBC factor of 1.02 mm/MHz tells us that the SBC to be added to the BC must be
(405.5 - 403.6) MHz x 1.02 mm/MHz = 1.94 mm

(corrected using right SBC factor for 403 MHz instead the 432 MHz number / 2014-07-10)

Boom Correction for mounting elements on boom with standard plastic insulators

The 8 mm element geometry data are to fit On-Boom-with-Standard-Insulators style (WiMo, Nuxcom, ...) of building.
Segmentation BC and Base BC (see below) must be added.

Ready to cut elements table for 20 x 20 mm Boom, 8 mm ele., 10 mm dipole, offset first / last ele. to boom ends = 30 mm

Remember this is UHF. Cut elements very precisely. Use a large calliper gauge if you can get one. Make a mark
for final length, saw about 0,5 mm in front of the mark and grind down to final measure. Below you find BC numbers for
other boom dimension

             Tube dim.          BC            SBC          BC+SBC
Boom        20 x 20 mm        7.1 mm    +   1.94 mm   =    9.04 mm
Boom        25 x 25 mm       10.4 mm    +   1.94 mm   =   12.34 mm
Boom         1 x 1  in       10.7 mm    +   1.94 mm   =   12.64 mm

Elements through boom

For building with elements through boom see DL6WU/G3SEK Boom Correction Online Calculator 
Do not miss out on adding the 1.94 mm of SBC.

Advanced feeding & symmetrising

Placing a ferrite an the feeding coax as close a possible to the dipole connection will do for any reception purpose
However - here is an advanced methode:
A simple symmetrising member may be made from a 3 x 1/4 Lambda line grounded at the far side with
N-flange-bushing and an aluminium plate and ferrite added as close as possible to the DE, see below.

Pattern and VSWR Plots

Elevation and Azimuth plot at 432.1 MHz (horizontally polarised - for vertical polarisation swap views)


SWR plots - inner band and full band


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EZNEC file of this Yagi with 8 mm elements  

Mounting a long vertically polarised Yagi

An ordinary boom strut may be used to produce a fixed distance to the pole. Which ideally should be a fiberglass
one. With help of a traverse and V-shaped kevlar, aramid or polyester guy ropes one can avoid boom sag
and add additional stability as now the supporting structure becomes 3 dimensional.

Place strut ends and guy rope fastenings as far away from elements as possible.


Stacking Dist.    DL6WU Formula
v-pol               1.31 m

Data of 2 Yagi stack using DL6WU stacking distances

Gain vs. isotr. Rad.  19.0 dBi
Gain vs. Dipole       16.8 dBD
F/B                  -28.5 dB
F/R                  -26.4 dB

Theoretical numbers, no phasing line losses
nor imperfections caused by H-frame included
*) T_sky = 200 K, T_earth = 1000 K as in VE7BQH G/T table

Azimuth Plot of 2 x vertical polarised YBN RS-14w
(Elevation Plot is same as Az. Plot given for single Yagi in h-pol above)

Symmetrising 50 to 50 ohms Feedline

The principle is similar to the 1/4 Lambda coax. Adding 2 x 1/4 Lambda or a half wave line does not change anything but allows
to form a gentle bow below the boom or until behind the Reflector. Follow practical construction hints on "Building a Yagi" page.

    Attenzione!     Take care when lengthening the coax, measure the right length instead of refering to given v-factors only.
    A good choice may be the diam. 5 mm PTFE coax RG-142 B/U: real resonate length (403 Mhz as 3/4 Lambda) shield-shield
    is around 373 mm

    Find more information  about getting the coax stub right on my Phasing & Matching Lines page

73, Hartmut, DG7YBN

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