Monday, May 4, 2020

Beverage Remote Switch

With two reversible Beverages for low band receiving -- north/south and northeast/southwest, and more planned -- I need a way to select the direction. I chose a remote switch with a DC control cable back to the shack. This is the more economical and convenient than multiple bringing coax from each antenna back to the shack.

The switch is uncomplicated to design and build. I brought the project forward due to the COVID-19 lock down even though the low bands are not a priority during the summer atmospheric QRN. Without access to helpers and the rapidly approaching peak hay and tick season the big antenna projects must be delayed a few months. While that's unfortunate it is no reason to slack and I am having no trouble keeping busy.

The remote switch is located at the edge of nearest hay field close to the feed points of two Beverages. Some separation is required to reduce risk of pattern degradation from conduction between grounds at each feed point and the switch.

To reduce the work I decided to use a buried and unused Cat5 cable (8 conductors) terminated at the 43 meter tower. The tower is about 25 meters from the Beverage switch so a trench across the field was dug with a shovel and pick, a Cat5 cable buried and the trench refilled. It took about 3 hours of manual labour. As exercise goes it isn't ideal but it'll do to keep me active and in shape.

In the picture but you can see that the trench is not particularly straight! It doesn't need to be and I wasn't careful about it. The post for the remote switch is just left of centre near the tree line. Near the right edge is the feed point of the 175 meter long northeast/southwest reversible Beverage. 

The cable is already connected through to the house so all I had to do was make connections at each end and at the tower junction box. Another Cat5 cable from the basement patch panel puts control of the Beverages on the operating desk.

The switch is so straight-forward a schematic isn't necessary. Indeed I didn't draw one; I had the circuit imagined in my head and went with that alone. I chose the smallest sealed metal box from my inventory that would fit the components. The layout for the holes was done on masking tape and then drilled. The mechanical fittings were installed and wiring proceeded.

Unlike the plastic boxes for the Beverage and its ports kept at different RF ground potentials this box must be metal and with no conductivity gaps. The purpose is to route common mode current on the Beverage connections to ground and allow none of it to seep into the interior. The ground lug on the left wall connects to a 4' ground rod below the box. Common mode at each Beverage feed point is similarly managed.

Control signal DC ground is tied to the Beverage antenna ground. DC ground and all signal lines have RF chokes for RF isolation from common mode currents from nearby transmit antennas. Other than the 1000 μH choke on ground line 'G' the chokes are the smaller 100 μH size to better fit the available space. Construction is dead bug style, with rigidity by dint of solid wires throughout.

The barrier strip may seem an odd choice for such a small box. Using a connector would require a larger box for the switch and soldering connector pins out in the hay field. The screws of the barrier strip make it easy to wire the Cat5 cable in the field and disconnect for maintenance or upgrades. The barrier strip does double duty as a mechanical anchor for the chokes.

The relays are SPST reed relays with integrated protection diodes. The coils use the inner 2 pins, with positive on top. The diodes make the coils polarized so orientation matters. One relay is powered at a time to select one Beverage. No antenna is selected without power. One SPDT relay can be substituted if an unpowered selection is desired. The SPDT relay must be energized (the default antenna deselected) when a different antenna is selected. Port-to-port isolation is good despite the back sides of the relays being tied together. This technique is only advisable for low frequency applications.

When two receivers use the Beverages they share a common feed line. There is no provision to have each receiver connected to a different Beverage direction or for diversity reception on a single receiver. Indeed the reversible Beverages have one feed line rather than the two required for sharing. Splitting the signal will result in a -3 db level reduction, or worse depending on how splitting is done. For my station I expect the Beverages to be used primarily on 160 meters and only occasionally on 80 and 40 meters when directional antennas for those bands need an RDF boost for the weakest signals.

An amplifier is a future consideration to equalize the signal level with the transmit antenna, recover gain lost by splitting or to equalize receive antennas. The longer a Beverage the higher the gain. There is a small difference between the 150 and 175 meter long Beverages. Other antennas may exhibit greater gain differences.

The 'R' control line places DC on the Beverage coax to power its relays for direction reversal. Because only one Beverage relay is powered at a time only that antenna receives reversing current. A choke and capacitor keep DC out of the feed line circuit. If a Beverage antenna is not reversible it requires a capacitor on its output transformer to prevent a DC short in case the 'R' line is inadvertently powered.

The F-jacks for the RG6 to the Beverages require a ⅜" hole and spaced far enough apart to provide room for a wrench (nuts) and for fingers (attaching coax). The hole for the feed line jack is ½" to accommodate plastic spacers (with a raised inner lip) that isolate it from the box -- more on this below. The hole on the far left is the minimum size to pass the Cat5 cable. The several small holes are for drainage. From experience some tiny insects can get inside. If the holes were any smaller drainage would be poor.

The feed line is isolated from the chassis and Beverage coax by the aforementioned spacers on the jack and with a 1:1 transformer. The antenna winding is between the selected Beverage (capacitively coupled) and antenna ground. The transmission line winding is between the RG6 shield and centre conductor. The extra bits of wire were snipped after the picture was taken, and tape was placed under the capacitors for additional shorting protection.

Here's the switch at the installation site with the control lines connected and partially dressed. The wire debris you can see was removed later. Although there are connectors for 4 Beverages antennas all 6 selector lines are connected. Other receive antennas could be added in future by knocking holes on the right wall of the box.

After testing with clip leads in the shack to confirm that the switch works as it should I added wiring and switches to my manual antenna control box on the operating desk.  The toggle switch selects the normal (top) or reverse (bottom) direction of the rotary selector.

I am planning a software controlled ergonomic receive antenna selector as part of my migration to station automation. As with some other projects it may be pulled forward if I cannot make progress this spring and summer on big antenna jobs due to helpers being kept away by "social distancing".

For now I am very happy with the instant direction switching, even it is just 4 directions. It is enough to be exceptionally useful. The small number of directions is only a significant problem on 40 meters where the Beverage main lobes are narrow. This will be remedied when I add more directions, as soon as late this fall. All I need to do is add the Beverages and plug them into the now existing switching system.

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