Loading link coupled final on 160

[k4kyv]

Link coupled rigs may not readily load into a 50Ω coax line on 160, using the conventional series or parallel  resonating capacitor with the link, without a special link  that would have too many turns for 80 and 40.  I was able to get mine to load up on 160 thru 40 (and even on 20) using the same 4-turn link.

This  might be of interest to those using Globe King 400's, BC-610's or homebrew rigs with link coupled output tank circuits.

The first page shows the basic concept, using series or parallel tuning with a capacitor to resonate the link.  It works well on 80 and 40, but on 160 even with the link fully meshed with the  coil, it would only load up to a fraction of full output.  I came up with a solution by trial and error, using a modification of the parallel resonant circuit for the link.  The resonating capacitor is split into two fixed capacitors in series, with a third variable capacitor to tune the circuit to exact resonance.  It serves as an rf voltage divider, with the coax coupled to only a portion of the tuned circuit.  This allows the link to see a coupling impedance that allows for a higher Q tuned circuit feeding the coax line.

The capacitors are large transmitting micas, rated at 6 amps rf and several thousand volts, although these may be overkill.  The variable capacitor is a  dual section capacitor, rated at about 2000 volts, somewhere between 600 and 900 pf per section.  It came from a military surplus BC-339 transmitter.  In my other tuner, I use a slightly smaller capacitor picked up at a hamfest.  In each case, the link is 4 turns.  The variable capacitor serves as a trimmer cap to achieve exact resonance at the desired frequency.  The circuit works over the entire 160m band.

I copied the circuit to build a second tuner for the other homebrew rig, and it worked first time I tried it.

The second page shows the circuit layout for 160m and for 80/40m.

The third page shows the actual schematic.  In one of my tuners the switch is a 4PDT knife switch.  In the other it is a ceramic rotary switch.  The antenna changeover relay is also shown.  The lead with the meter in series goes to the centre-conductor of the coax.  The 4 circles with the lines connected, just to the right of the switch in the schematic are screw terminals on the variable capacitor.  The two sections are connected in parallel by strapping the outer screws.  The wire going to one of the inner screws connects the circuit at the switch to the two capacitor sections.  Scroll four messages down to see photos of both the matching units.

This message was inspired by Steve's GK 400 on 160m thread.

[KA8WTK]

Hi Don,
  I got the series tuned link to work on my 813 rig. I can't remember the value for the variable, but I had to put a door knob across it in order to get it to where it would tune the link into the 50 ohm coax. It does work, but the "mileage may vary" in other situations and it may not be "ideal".

Bill KA8WTK

[Steve W8TOW]

The problem with the GK400 isn't necessarily that the FINAL won't load on 160m,
but rather there is a problem coupling to the GRID of the FINAL on 160m...
Now, this probably is still the same LC issue as previously discussed regarding the
PA tank side, but also on the GK400's there is a .001 uF cap in series to GROUND
with the SWINGING LINK of the JVL GRID INPUT coil.
In the Military TECH manual, there is reference to MODIFYING the LING of the
GRID coil in addition to adding a PADDER Cap across the split coil to compensate
for the lack of "C" in the GRID tuning circuit....
Last EVE, I removed the JVL GRID coil and made a coax jumper to plug into the
socket of the JVL GRID COIL LINK... I then ran this coax to a  dummy load...
It was right on 1885 KC and the 807 was delivering about 15 watts of RF!
SO, at least the OSC & BUFFER stages are running on freq!
73
Steve
8TOW

[k4kyv]

My two homebrew transmitters use link coupling, both for the final amplifier output circuit and the driver-to-grid circuit.  I use a swinging link at the driver plate tank coil, and a fixed link at the PA grid coil.  The swinging link is used for grid drive adjustment.  I have never used series or parallel capacitors with the driver-PA link circuit.  On one rig, the driver is a separate unit,  so I use a few feet of coax to connect the two links together.  The other rig is self-contained, and I simply use a twisted pair made of stranded hookup wire to connect the links.  The PA grid and driver plate coils in each set are nearly identical, with 3 or 4 turns on each link.

To tune up, I run the link at the driver plate coil out to minimal coupling, and dip the driver stage.  Then I increase the link coupling just a little, and tune the grid circuit for a slight peak in the driver current.  When I am sure the driver stage is perfectly dipped and that the grid circuit is perfectly resonant as indicated by the peak in driver plate current, I increase the  link coupling until normal grid drive is achieved.  Then I re-check the driver plate for dip, and the PA grid for resonance.  Usually, no additional tuning is necessary.

I have always been able to get more than sufficient coupling between stages on 160, 80,40 and 20, by merely connecting link to link, with no additional tuning elements in the circuit.

Before I put up the tower, my antenna was an 80m dipole strung between two trees, with the open wire tuned feeders running directly into the shack, at that time a spare upstairs room in the house.  I feed the open wire with a link coupled balanced tuner mounted on the wall, with plug-in coils for each band.  The link output from the transmitter was connected to the link input to the tuner, using heavy gauge zip cord with the conductors separated by pulling them apart.  Likewise, I was always able to get plenty of coupling with 4 turns on each link, on 160-20 metres.  I devised the matching network described above when I used RG-8 to feed a quarter-wave 160m inverted-L located some distance away, and I still use it to feed the present antenna system.

[k4kyv]

Here are photos of the two rf output matching units used in the circuit described above, that couple the link output from the homebrew transmitters to the 50Ω coax line that runs out to the main antenna tuning units located at the base of the tower.

[Steve - WB3HUZ]

Yo. Those are some big components for match the driver to the grids of the final! What kind of maul are you running there Don?

[k4kyv]

Those are not the driver tuning networks.

Those are the matching networks as shown in the schematic drawings in my initial post on this thread.  They match the link coupled final amplifier output to the 50Ω coax line that runs down to the tower, to feed my main antenna tuners located in a sheltered enclosure at the tower base.  The 110' high dipole is fed with open wire line from the tuner, running up through the interior of the tower.  On 160, the insulated base of the tower is fed from the coax line through an L-network.

[Steve - WB3HUZ]

Yea, yea. Now we know why you have that BIG signal.

[flintstone mop]

Ya can't beat a real antenna!!

Fred

[N3DRB The Derb]

Don's rig is uber cool. and BIG. lotsa neat early iron too.