9:1 un-un for HF receive only
The Rotorua Branch 33 , 2M Linear repeater site contains
our Icom R75 HF receiver ,Remote controlled by DTMF, The R75 has two
antenna ports .A standard 50 ohm SO239 and a 500 ohm random
wire "long wire"connection. I had modified the DTMF control
software such that the two antennas could be selected from the any DTMF enabled
hand held on 145 Mhz, selecting either A or B from the radio's 4X4 keypad .
Very recently one of the local club members craig ZL1ER obtained by fair
means or foul, a ex CB "ringo" vertical antenna for
26-27 Mhz ( half wave dipole fed at the base by a single turn tapped tuned
circuit with an unbalanced 50 ohm connection. )
He duley erected the antenna on a 6 M support pole behind the water tank near
the hut and we ran the low loss coaxt
into the repeater hut . At the moment the HF dipole connects directly to
the SO239 connector , we had to connect the retuned 10m vertical to the
receiver so I connected the ringo to a 4:1 balun and then the two öpen
wires"to the 500 Ohm port on the receiver . The antenna selection was
tested and the 10 vertical showed itself much better at receiving 10m signals
than the HF dipole .
However on further testing over the following days
I discovered that the 10m antenna was receiving over a much wider
bandwidth than it should have , we could even receive the wellington
mountain radio service on 3345 KHZ USB on their morning skeds. This prompted me
to look at the connections of the 4:1 Balun supplied by Ed ZL1AVV
and I saw that the use of a balun in this configuration , enabled the
whole coax and antenna to work like a miss connected random wire. A re hash was
needed if we wanted it to perform well . Correct matching of the 50 ohm
coax to the 500 ohm connection was needed and this would be acomplished with a
1:9 Unun ( unbalanced to Unbalanced) So I trolled the internet for
recipies, Fired up the trusty E5062A VNA and set to work in the shack .
The 1:9 RF transformer had to meet a few important criteria ,namely HF response
to 26 Mhz and down below broadcast band as well
and an acceptable loss . Initially I was going to leave the HF antenna attached
to the 50 Ohm input and connect the 10M antenna through the matching unun
to the 500 ohm random wire input . How ever after some thought i decided to put
the 10m antenna on the 50 ohm port and the HF antenna via the Unun on the
500 Ohm port as we could put up with a larger signal loss of
the HF dipole due to atmospheric noise than the more quiet spectrum
up at 28MHz.
I Shagged around all day on that Unun, winding bifilar,trifilar .colour
coding wires tinning wire .fat torroids thin, long short .. binocular
large small . The problem is having a piece of test kit to look at
device performance pushes you more to the perfectionist. I
could see what was going on and how well the unun performed or didnt as the case
would be . I could get wide
banwidth but not down to below broadcast band , lumpy response . unacceptable
loss , . the other problem with making these ununs is that I had to make
two identical ! the analyser is 50 ohm input so you have to connect the unus back to
back and take half the total through loss as the performance of one
unun.
Ferrite bead "onion" connection back to back
In the end i decided to try some 4:1 ununs I had made some months earlier ( 5 turns) so I dug them up and swept them and was pleased with the result .low loss and semi acceptable bandwidth , the low frequency response depends on the permiability of the ferrite medium and the inductance of the windings . Cant change the permiability but i can the inductance, so i wound up some trifilar beads this time, with with 7 "turns" to push the low frequency response , connected the windings for a 1:9 unun and swept the two ununs back to back ... perfecto ! an acceptable result. broad band , low loss and excellent low frequency response. Ferrite beads are manufactured to becom lossy above a certain frequency depending on the chemistry of the ferrite. The ones I used turned to custard as a transformer above 100 Mhz .through loss increased and the passband went lumpy . but that was not an issue for my HF requirements .
9:1 ununs sweept back to back
This is exactly what is required , Good low
frequency response (0.6 dB) at 360 Khz , below the frequency
to what the dipole is cut and low attenuation 0.42 dB up to 28 Mhz
, ( divide graphical attenuation by two as there are
two ununs back to back )
450 ohm
connection
trusty so239 50ohm
inside
box
close up of single bead
I mounted the unun in a small enclosure made from RHS aluminium the black earth terminal is connected directly to the metal hemce the length difference
next test is to install it on the radio and see what happens
regards mike ZL1BTB