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 loop’s 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 system’s 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:
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 KVp-p at the coil tap points.
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 power.
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 phase.
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 microHAM’s 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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