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Default [AB1OC/AB1QB] An 80m Broadband Matching System

Our HAM Station

An 80m Broadband Matching System

Posted: 28 Jan 2018 03:09 PM PST

Our Tower with 75m Loop

We installed a 75m loop for SSB operation on our tower when we built it.
The loop is full size and is diamond shaped so that our lower SteppIR DB36
yagi can rotate inside of it. The loop is fed at the bottom corner about 20
ft up from the ground. It works great for SSB operation on 75m but we have
often wished we could use it across the entire 80m band. This goal led to a
project to create a matching system for the antenna. The idea was to use a
set of loading coils in series at the feed point create a good match in all
segments of the 80m band.
EZ-NEC Model for 75m Loop

The first step in the design of our 80m matching system was to build a
model of our current loop using EZ-NEC. The model was then used to
determine the correct values of a set of series loading inductors to match
different segments of the 80m band.
Matching System Design Analysis

We also considered how likely different segments of the 80m band were to be
used by profiling historical spotting data from DXSummit. All of this
analysis led to the creation of a final design which is captured¬*in the
spreadsheet shown above. The final design requires our current 75m loop to
be shortened a bit to work well at the very top of the 80m band.
Modeled Loading Coil Inductance Values

A set of 5 different inductor pairs can be used in series with the loops
feed point to create a good match across the entire 80m band.¬*The modeled
values for the series matching inductors is shown above.
Matching System Modeled SWR

Our microHAM control system can easily implement the switching of the
various inductance values based upon the frequency that a radio using the
antenna is tuned to. Result modeled SWR for the final 80m loop and match
combination is shown above. The design should achieve an SWR 1.5:1 across
the entire 80m band except for the very top where the SWR remains 2:1.
Also, the design optimizes the systems SWR in the important CW DX, SSB DX,
and Digital windows on the 80m band.
Layout of Components in Enclosure

With the design completed, we choose an enclosure and all of the
components. Here are the details of what we used:

Barker and Williamson 2404TL 32 uH Airdux coil stock cut in half (3 dia, 4
Array Solutions RF-15 15/20 kW RF Relays
Balun Designs 1.5:1 Balun with eyelets
V22ZA2P Varistors (MOVs), 0.1 uF Capacitors, 1N4001 Diodes make up the
12Vdc relay control circuits
Outdoor NEMA Box Approx. 16x12x6.5
DX Engineering Saddle Clamps to fit our tower

The first step in the construction was to layout all of the components in
the enclosure. Attention was paid to keeping the two series inductors at
right angles to avoid coupling and to keep RF connections as short as
possible. The relays were arranged to keep the leads connecting to the
coils of roughly equal length. Finally, the control circuitry was kept as
far removed from the RF leads as possible.
Enclosure Mounting Ears and Clamps

The matching system attaches to a tower leg via saddle clamps. We
fabricated a set of mounting ears and spacer blocks to position the
enclosure¬*far enough away from the tower so that the antenna connections do
not interact with the tower.
80m Matching System Construction

A summary of the completed matching system construction is shown above.The
design uses a set of four double-pole double-throw relays to switch in
different coil taps which selects the loading inductance provided by the
matching system.

We did a set of calculations and found that our relays would be subjected
to a worst case peak-peak voltage of about 2.1 KVolts at the coil tap

The relays are arranged such that two sets of contacts have to be traversed
to select any given coil tap.¬*The relays we are using have a third pole
which we are not using. We disassembled each relay and removed the internal
contact wiring for the center pole which improves both the coil to contact
voltage rating and the isolation values of the relays.

These steps combine to improve the voltage rating of the system. This is an
important design element given that the match will operate at legal limit
Completed RF Deck

The completed RF deck and control circuitry is shown above. The enclosure
we choose came with a removable plastic plate that made mounting and wiring
all of the components simple.
Loading Coil Mounting and Taps

The loading inductors are mounted using nylon hardware with the ends
connected to the two antenna terminals on the sides of the enclosure. The
coils use movable tap clips to allow us to fine-tune the match once the
system is installed with the antenna on our tower. The initial clip
locations are set to create the inductance values modeled during the design
Relay Control Circuit Connections

The relay control leads use twisted pair wiring to minimize RF pickup. The
control leads are routed away from the RF connections to minimize potential
RF coupling.
Relay Control Circuit Details

The control circuits for each relay use a combination of a Diode, a
Varistor (MOV) and a filter capacitor in parallel to avoid relay coil
switching interference and to suppress control line noise.
1.5 to 1 Matching Balun

The matching system is designed to operate at 75-ohms which is pretty close
to the resonant impedance of our 75m loop. The current antenna uses a 1.5:1
Balun to match the loop to our 50-ohm coax feedline. We disassembled an
identical matching balun (actually a 75-ohm balun plus a 1.5:1 unun) and
used it without its enclosure to create a final 50-ohm match.
MicroHAM Setup to Control 80m Matching System

The final step in the construction of our matching system was to program
our microHAM antenna switching system to properly configure the relays in
our matching system. This was quite simple to do using microHAMs frequency
dependent antenna control capabilities. The microHAM system automatically
operates the appropriate relays to create the best possible match as the
radio which is using the matching system is tuned across the 80m band.

Unfortunately, we are in the middle of winter here in New England so I will
have to wait for warmer weather to install our new matching system on the
tower and make the final adjustments. I am planning another article here
when the final integration steps are done to cover the performance of the
completed project.

Fred, AB1OC

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