DG7YBN / 432 MHz / GTV 70-25m
  Last Update Oct. 11th 2016




.......



Performance Data and Geometry
Pattern and VSWR
Download as File
This Yagi with bent Folded Dipole
Stacking
Matching


GTV 70-25m Yagi   with bent Driven Element

EME + SSB band version

This Yagi has very low back lobes for its length. It may serve as single antenna for Tropo or even EME.
A 4 Yagi bay will make an excellent EME array. It also makes a quiet contest antenna due to its high F/B.
The bent DE (K6STI style) transforms from approx. 17 ohms to 50 ohms at feed point.

Current distribution





Take a look at the 2 x GTV 70-25m built by Sverker, SM7THS
and what he reports abt. operating EME with these on QRZ.com



Almost ready built GTV 70-25m at PA2CV, Alex

Below, on left: Build completed and ready to hit the moon 2016-10-15, click on image to view in large
And 2016-10-16 on right: First test as EME novice with 100 W out, instantly an almost QSO with PI9CAM due to much drift in trx.

   


Slideshow: Ingenious bent dipole for the GTV 70-25m by Alex, PA2CV. 8 mm soft copper tube with brass screws in the tips





Performance Data

Gain vs. isotr. Rad.  18.9 dBi (4 mm elements and low segmentation)
Gain vs. Dipole       16.8 dBD
-3 dB E-plane         22.2 deg.
-3 dB H-plane         22.8 deg.
F/B                  -36.9 dB
F/R                  -34.0 dB
Impedance               50 ohms
Mechan. Length        5820 mm
Electr. Length        8.39 λ

Stacking Dist. h-pol.
top-to-bottom         1.76 m
side-by-side          1.80 m




Geometry

EZNEC wire table for Ø 4 mm elements




Table 1: GTV 70-25m, 4 mm elements through boom:



"Ready to saw and drill" data for mounting elements through boom with BC according SM5BSZ's BC.exe:
Note: with through Boom BC it is important to stick to the boom end offsets given below!

This table is only valid for:
Boom shape: square
Boom dim: 20 x 20 mm
Wall thickn.: 2.0 mm
Holes in boom: 6.0 mm
Offset rear: 30 mm
Offset front: 30 mm

    <=   NEC Geometry 4 mm ele. insulated through boom incl. SM5BSZ BC.exe Correction





Table 2: GTV 70-25m, 8 mm elements semi insulated on boom with standard holders:


    <=   NEC Geometry 8 mm ele. semi insulated on boom incl. DG7YBN BC and an SBC of 1.37 mm




The Drivers diameter is 10 mm for all examples. Use EZNEC's Auto-Segmentation at 1050 MHz.

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.

Sketch of Bent Dipole










Pattern and VSWR Plots

Elevation and Azimuth plot at 432.1 MHz

 


SWR and Return Loss plots - simulated with 4nec2


Return Loss plot - PA2CV's build: 430.0 to 435.0 MHz (tnx Alex!)










Downloads

EZNEC file of this Yagi with 4 mm elements  

EZNEC file of this Yagi with 8 mm elements  











GTV 70-25m with Folded Dipole


The straight split bent dipole can be replaced with a bent folded dipole like in any non-bent Yagi design.
In doing so the transformation ratio of 1:4 remains. The bent folded dipole versions impedance is 200 ohms.

The folded dipole itself is of Ø 6 mm; I recommend an 'elements through boom' built.
So that the folded dipole is centered in the element plane for best symmetry in the elevation pattern.

Geometry of the Folded Dipole

In the model we use the center of the tube, regardless of its actual diameter.
With this height is 44 mm, span width is 308 mm. Adding the real diameter of 6 mm we end up with

inner height = 38 mm, outer span width = 332 mm, see sketch



Note     Folded Dipole mounting in plastic brackets compensation

Whatever plastic block is used to suspend or mount the dipoles wires will need to be compensated
by adding ~ 0.1 mm per millimeter of wire running in plastic. If you attach a 20 mm plastic block on top
of the boom to lead the dipoles upper wire a length of 2.0 mm / 2 = 1.0 mm needs to be added to the dipoles
span width. Assumed it runs free of contact to plastic on the down side.
We use factor 1/2 in a folded dipole then since it is a full wave loop.

Same applies to any BC if the wires run very close (~ < 3 mm) to the boom.

SWR and Return Loss plots - simulated with 4nec2









Stacking

As on the 432 MHz Band the Y-factor = T_earth / T_sky is that high I see little chances in
bettering an array's RX performance by using "Over Stacking" distances. However, depending
the level of local QRM it might be worthwhile to try less distance, especially in H-plane.



Stacking Dist.    DL6WU Formula
top-to-bottom         1.76 m
side-by-side          1.80 m

Azimuth plot and data of 4 Yagi bay using DL6WU stacking distances


Gain vs. isotr. Rad.  24.84 dBi
Gain vs. Dipole       22.69 dBD

T_ant                 26.4 K*
G/T                  10.63 dB*

Theoretical numbers, no phasing line losses
nor imperfections caused by H-frame included
*) T_sky = 20 K, T_earth = 350 K as in VE7BQH G/T table



Elevation plot and data of 6 Yagi bay using DL6WU stacking distances

Gain vs. isotr. Rad.  26.58 dBi
Gain vs. Dipole       24.43 dBD

T_ant                 26.1 K*
G/T                  12.42 dB*

Theoretical numbers, no phasing line losses
nor imperfections caused by H-frame included
*) T_sky = 20 K, T_earth = 350 K as in VE7BQH G/T table










Symmetrising 50 to 50 ohms Feedline to 432 MHz Bent DE

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 (432.2 Mhz as 3/4 Lambda) shield-shield is around 348 mm


  Find more information on Phasing & Matching Lines page






73, Hartmut, DG7YBN


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