edited and amended11/02/2009, kaz
Here is a small project that will work along with the K9AY RX antenna, and solve a minor SO2R problem in the KazShack.
Currently theK9AY feedline comes into the KazShack directly to the RX input for one of the transceivers. I wanted to have a way to share the antenna between the two radios without connecting the RX inputs of each rig directly to one another(RF isolation), or manually swapping the feedline between radios.
There are some comments in various places about using the W3LPL RX bandpass filter design to split the bands to multiple destinations. The NCJ article “Distributing Receive Antennas” by K3NA and W2VJN is a very handy and well explained reference.
This was also desirable here in the KazShack, which sitsin the shadow of the 50KW WPTF on 680kc broadcast transmitter. Rolling the W3LPL filters was done using some T-50-3 toroids and NP0 and high accuracy monolithic ceramic capacitors from the parts bin. The filter is built dead bug style. Each band is on opposite sides of a single piece of copper clad board. Basically, the input is fed to each of the filter banks, and the 160m and 80m bands come out separately, each isolated from the other.
The coils for each band are identical within the band(i.e., L1=L2=L3), so after winding each I used the MFJ-259 to resonate each coil to the same frequency using the same capacitor. After soldering everything together, a quick test with the antenna analyzer into a dummy load showed each section to show minimum SWR right where I wanted it. No other tuning was required. Almost too easy.
Left alone at that point, I could feed either radio from either band, but there needs to be a switch of some sort to eliminate the need for swapping coax feeds during the heat of the action. This appeared to be another ideal application of the small signal relays that were also on hand. Using a single DPDT relay, the filter outputs can be switched between the radios with the flip of a switch. A toggle switch mounted on a remote panel is used for convenience . Simple but effective.
The remote panel is a smallsection cut from 1’5 inch(about 38mm) aluminum angle stock. I pre-drilled pilot holes for future use, and installed the switch for the RX antenna splitter, as well as a control for a planned 40m remote antenna switch. The “panel” is then attached to the inside edge of my home brewed fold out station cabinet. The cabinet is filling up fast – not much room for any more equipment in there.
Using the switch is going to make swapping the low bands from one radio to the other a snap. Literally as easy as flipping a switch. The band pass filters will also help isolate the radios from one another in the SO2R environment, as well as reducing the broadcast band harmonics.
There is a bit of signal loss in the filters, but probably not enough to be significant while operating. Hopefully the much lower noise levels on the RX antenna will offset these slight losses. I have not felt a need for an external RX preamplifier before now, but now I am looking at the ARR 1-30. It would be nice to boost the RX signals to parity with the noise on the TX antenna. That might reduce the amount of volume control “riding” needed when looking for the best RX on a contact when toggling between the RX and TX antennas.
Yet another fun little project. It isn’t as much satisfaction as growing an entire rig from scratch, but it is always fun to put a useful bit of home brewed kit into action.
PNG image of schematic for W4KAZ’s version of the “W3LPL RX band pass filters” built as an antenna splitter and switch.