have built a number of antennas over the years - first using yagi
designs from the handbooks, then using the NBS designs, then a log yagi
from ARRL, then a design by DL6WU. The latest for 4m is a DK7ZB
6 ele yagi, a nominal 28ohm design, very slightly modified to take
account of half inch elements instead of 12mm OD. The dimensions are as
The analysis of its performance using MMANA
is shown below.
My yagi was kindly built for me by Phil G3TCU who
has an article describing (amongst other things) its mechanical construction here
Building an antenna
For suppliers of aluminium tubing and booms in UK, try Aluminium Warehouse
or in south east England there's Rudgwick Metals
You can also order parts including tubing and element clamps from Nuxcom
in Germany. Nuxcom are also selling kits to make up 4m and 6m
antennas to DK7ZB designs.
Stacking 4m and 6m antennas
to have both 4 and 6m antennas on the same mast, I was interested in
the possible interaction between them. The antennas are the
6 ele DK7ZB yagi described above, and 5ele 6m M squared yagi. I
used MMANA v220.127.116.11 to model them, and to find out what
deterioration there might be in forward gain and front to back ratio.
results of the modelling are in the table below. I've modelled the
antennas with the reflectors aligned (not very practical) and the
centres aligned (more practical) for comparison. The 4m yagi is
mounted above the 6m yagi. Gain comparisons are modelled in free
space to avoid any height effects due to ground reflections.
|MHz||Height separation||Gain dBd||f/b dB||Z|
|50.1||6m yagi alone||9.27||19.8||16.9+j11|
|50.1||1m, reflectors aligned||9.24||18.03||15.9+j10|
|50.1||1.5m, centres aligned||9.24||18.1||16.1+j11|
|70.2||4m yagi alone||10.14||20||27.8-j0|
|70.2||1m, reflectors aligned||9.57||13.26||19.8-j4|
|70.2||2m, reflectors aligned||10.09||18.7||26.2+j0.2|
|70.2||1m, centres aligned||9.66||14.1||23-j6.6|
|70.2||1.5m, centres aligned||9.94||16.5||25-j1.9|
|70.2||2m, centres aligned||10.09||18.5||26.5-j0.5|
The modelling shows the effect of a 1.5m separation as
on 6m, 0.03dB loss of gain and 1.7dB worsening of f/b
on 4m, 0.2dB loss of gain and 3.5dB worsening of f/b
have done some measurements using the nearest beacon on 4m (G4JNT/P)
and the GB3MCB beacon on 6m (not close, but the best for this purpose).
best f/b ratio on 4m before stacking was 22dB, and 20dB after.
There was no detectable difference in forward gain (less than
On 6m, the f/b ratio after stacking was again 20dB. These ratios are both better than the modelling predicts.A cautionary note
- when running 400W on 6m with this layout, there is about 0.4W
returned on the 4m feeder, equating to about 4.5V. Similar
results apply when transmitting on 4m (160W out -> 0.1W returned on
6m feeder ~2.2V). Some protection of both receiver front ends is
advisable, although I have found no problem so far.
A 1.5m separation of antennas is satisfactory with no discernable degradation of performance.
Operating cross band - a 6m bandstop filter
trying to operate cross band I found an excessive amount of noise
entering the 6m receiver from the 4m transmitter. To overcome
this I designed a bandstop filter to go in the 4m transmitter output
feedline. Details are on the equipment page here
Adding a 6m vertical
There are several ways of adding a vertical antenna -
- a ground plane mounted on an additional 3m pole above the existing antennas
- a vertical quarter wave element using the 4m beam as its ground plane
- a vertical dipole mounted ahead of the support pole
these with MMANA showed that (1) had a gain of -0.2dBd, a low impedance
~19ohms, and an elevated polar diagram with a peak response at about
28 degrees above horizontal. Slightly better was (2) with a gain of 1.3dBd, but too
high an impedance (153ohms), and too much horizontally polarised component.
(3) I used a driven element 0.9m ahead of the support pole and length
2.76m. A half reflector length 1.54m is added above the support
pole - this gives a gain of 2.6dBd, front to back of 4dB, and
impedance of 67ohms. Horizontal radiation is more than 16dB down.
The polar diagram is shown below. The vertically
polarised pattern is shown in red, horizontal in blue.
an impedance of about 68ohms there seems little point trying to match
it any better since direct use of 50ohm feeder will give a VSWR of
1.34:1. It is however desirable to use a balun, and a quarter
wave balun was chosen - see eg ARRL antenna book page 26-26. RG58CU has
a velocity factor of 0.66 so for 50.16MHz the length of balun
For a photo, see the diversity
interaction between the 4 and 6m yagis and the vertical was analysed
using MMANA. There was no change in gain, front to back ratio, or
impedance of either yagi.
After installation, the VSWR measured
1.1:1 at 50.0 and 50.16. I measured the power being received on
the vertical when transmitting on each yagi. There was 0.1W received
when transmitting 160W on the 4m yagi, and less than 0.1W when
transmitting 400W on the 6m yagi. These values indicate good
orthogonality and a good measure of protection to a receiver connected
to the vertical.