ladder line PTT and TR switches Oh My

[W7SOE]

The saga of my AM station continues, ever so slowly.  I have constructed a PTT relay circuit using a voltage doubler (6.3VAC to 12VDC) for the JVII.  I almost ready to replace the coax feedline with ladderline, I just need a couple hours on the roof which may be hard to come by.

I am using an R-388 for the receiver, and a JVII feeding a Matchbox.  The Matchbox is all back together.  After I add the PTT circuit to the JVII I plan on having the relay in the match box switch the antenna to the transmitter and mute the receiver.  Some questions:

Anyone have a connector idea for the 110VAC output from the JVII (Funny connector next to the line cord)?  This goes hot on transmit and will power the relay in the Matchbox and a TR relay.

The Matchbox relay will turn off the receiver B+ and ground the receiver antenna.  The antenna to the receiver will come from the TR switch, not sure how to ground it as it is coax...  Have to check if I can turn off B+ externally on the R-388...

Not sure how to power the TR relay.  Could be powered by the same 110VAC output from the JVII.  How to interface ladder line to a TR relay?


I know I am reinventing the wheel but this is my first trip through this.  Any advice/corrections are welcomed.

Rich W7SOE

[W1GFH]

Anyone have a connector idea for the 110VAC output from the JVII (Funny connector next to the line cord)?  This goes hot on transmit and will power the relay in the Matchbox and a TR relay.

That 2-pin thing? Someone was selling them a while ago (see below) but you can JS one out of an old crystal with some filing of the pins. Some folks just make up a cable using two old VOM probe pins soldered to each side of a pigtail (using adequate heatshrink).

Not sure about the rest of your setup, sounds complicated. I'd disable the matchbox relay and go with a big, external 110VAC contactor relay. The JVII will trip the contactor on transmit and you can use one set of contacts to switch the antenna, another set to mute the receiver, and another set to do whatever else you want (such as trigger an "on air" light, or a rotating disco ball ). Coax for all antenna interconnections simplifies things. The ladder line from the antenna would only be attached to the balanced line "antenna" terminals of the matchbox.


*OK, Glenn Zook, K9STH has them at http://k9sth.com/Page_3.html -- HC6/U style plug for connecting antenna relays to many Johnson transmitters including the Valiant and Ranger. These come with 24 inches of stranded "zip" cord for connection to the relay.Price for the assembly:  $10.00 postpaid CONUS, $12.00 US postpaid Canada, $10.83 Texas residents

[W7SOE]

I finally received my 75m crystals for the JVII.  While the inverted V (cut for 40m) loads up great on 40m using the matchbox, I am having real problems getting it to load on 75m.
I did get it down to1:5 but it is very touchy and it seem that I have to add extra coupling in the Viking II to get the same plate current.
At one point I heard bad noises from the tuner (arcing?) when the matching was way out of whack.
So I guess I will try to find time to get rid of the coax antenna feed ( including the currrent balun at the feedpoint) and replace it with ladder line.  Hopefully this will help.

I put together TR switch.  I used a MASSIVE 40A relay.  It gives the right amount of Kahthwack! when powered.

73

Rich

[Tom WA3KLR]

Along the same lines (HI) I was thinking recently about the apparent fact that many AM’ers don’t seem to be able to hear r.f. signals very well.  Also, many AM’ers use the R-390/390A which has a balanced antenna input.  Many use a balanced antenna with a ladder line feed.  But I presume almost all are using a tuner that converts from the ladder line to unbalanced and use a coaxial relay in the shack to interface to the transmitter output and receiver input.

I am wondering if for some stations if bringing the balanced line all the way to the receiver input would help on receive, due to common-mode rejection of local noises.  I did picture using a large open-frame relay at the output of the tuner.  The feed line would come to this relay.  The tuner’s coaxial input would always be connected to the transmitter output. A balanced line of the same impedance (Z not critical, but probably best if shielded) and smaller spacing would go from the big relay to the receiver’s balanced input.  A separate relay would be used for muting/control.

One disadvantage is that the match of the tuner is lost, but on receive the receiver input Z is the load on the transmission line end (ant. feed point is the r.f. generator), rather than the antenna feed point impedance, and with the signal levels involved on the lower bands, this may not be a problem at all.

You can first connect the feed line to the receiver directly for test comparison instead of building this system first.

[W1GFH]

I finally received my 75m crystals for the JVII.  While the inverted V (cut for 40m) loads up great on 40m using the matchbox, I am having real problems getting it to load on 75m.
I did get it down to1:5 but it is very touchy and it seem that I have to add extra coupling in the Viking II to get the same plate current.
At one point I heard bad noises from the tuner (arcing?) when the matching was way out of whack.
So I guess I will try to find time to get rid of the coax antenna feed ( including the currrent balun at the feedpoint) and replace it with ladder line.  Hopefully this will help.

The simplest solution is to add a couple of 25 foot or so "extensions" to each leg of the 40M dipole. The extensions can be made of any old wire and can slant or droop down vertically to fit your space.

I put together TR switch.  I used a MASSIVE 40A relay.  It gives the right amount of Kahthwack! when powered.

The thwack is important, isn't it? 

[Tom WA3KLR]

I believe that the Viking II's output impedance range is a little limited.  They were expecting you to buy their Matchbox also.

But most transmitter pi or pi-L output networks becomes more limited in impedance range at the lowest bands due to the practical maximum capacitance chosen for the air variables.

Ideally a good antenna tuner should be able to match to your 40 meter antenna at 75 meters.  Since the antenna is electrically short at 75 meters, the feed point impedance will be relatively low.  As Joe says, changing the feed line length may modify the end impedance enough for your tuner to achieve a match.

Rich,

If you can give me an accurate description of your antenna system I can simulate the antenna and plug in the resulting feed point impedance to a transmission line program and give you a ballpark value for the impedance presented at the transmitter end of the feed line.

I need antenna heights and length, ladder line length, description of ladder line - impedance if known, wire size and spacing.

[W7SOE]

Tom,
   Yesterday I converted the feedline to ladder line (450 ohm, 18 AWG item #551 from the Wireman).  I am still having the same problem loading the antenna.  I can get the SWR down to less the 1:5 but it is tricky and then I can only load the till the plate current is ~180 mA.  If I load beyond that the tuner arcs over!

I am not sure how long the feedline is, I would guess ~60ft.

The antenna is an inverted V, 64 ft total length.  I would guess the angle is about 30-40 degrees down from horizontal.
It is mounted on a 15 ft metal pole on my roof so the feedpoint is ~35-40 ft up.

Thanks

Rich

[W3SLK]

Tom said:

But I presume almost all are using a tuner that converts from the ladder line to unbalanced and use a coaxial relay in the shack to interface to the transmitter output and receiver input.

Tom, one of things discussed either here or on the AM Window, was the fact of using a balun similar to that which is used on the back of a TV. Although there will be a slight mismatch, ie 75Ù vs. 50Ù and 300Ù vs. 450Ù it is very minimal. I as others have surmised that it will work FB.

[W1GFH]

Tom,
   Yesterday I converted the feedline to ladder line (450 ohm, 18 AWG item #551 from the Wireman).  I am still having the same problem loading the antenna.  I can get the SWR down to less the 1:5 but it is tricky and then I can only load the till the plate current is ~180 mA.  If I load beyond that the tuner arcs over!

I am not sure how long the feedline is, I would guess ~60ft.

The antenna is an inverted V, 64 ft total length.  I would guess the angle is about 30-40 degrees down from horizontal.
It is mounted on a 15 ft metal pole on my roof so the feedpoint is ~35-40 ft up.

Thanks

Rich

Trying to load a quarter wave on 75M will always result in "touchy" tuner settings, especially at higher power. As I suggested, adding 25 foot wire extensions to each leg of the inverted vee will fix the problem in short order. You can droop the extensions or zig zag them horizontally around the yard.

[Tom WA3KLR]

Rich,

Given your 40 meter inverted V dimensions, I came up with these results with the computer programs:

I see your 40 meter wires resonant at 7455 kHz., just for your information. 
Driven at 3885 kHz., the feedpoint impedance is 8.2 – j805 Ohms.  Running this load on 60 feet of 450 Ohm “Window line” selected from the transmission line program, I get 10.9 + j250 Ohms at the transmitter end of the line.  The program used a VF of 0.95 for the Window line.  Wireman site says VF = 0.91 for your #551.  So my results are just slightly different.

The loss for the 60 feet of transmission line is 6.3 dB.

If you add 20 feet of ladder line, the impedance at the transmitter end will increase to 36.7 + j746 Ohms.  At 100 feet of line from the antenna feedpoint, the impedance hits a maximum, which is to be avoided.  This distance would be 96 feet with VF = 0.91 line.
I wonder if your line is actually longer than you think it is?

Anyhow these are the numbers I get, which are a ballpark number guide for you to let you know what the approximate impedance situation is.

[Tom WA3KLR]

If  the feedpoint impedance at 75 meters had no reactance, such that the impedance was 8.2 – j0 Ohms, then the impedance maximum would occur at 60 feet as Joe mentioned.  But with the 8.2 –j805 Ohms reactive load, the impedance maximum occurs at 100 feet.

I added 25 feet of wire to each end of the dipole and the ends slope down to 10 feet above the ground.  The whole antenna is in one vertical plane.

For the new feedpoint impedance at 3885 kHz., I get 41.3 – j113 Ohms.  Seems real nice, but after traveling through 60 feet of ladder line the impedance becomes 626 + j1517 Ohms, a nasty high and reactive impedance.

[Tom WA3KLR]

I forgot to mention that the ladder line loss drops to 0.6 dB. with the new higher feed point impedance at 3885.  The loss was 6.3 dB. before with the original 40 m. dipole.

[W7SOE]

Tom and Joe,
    Thank you for the help.  It looks like I am fighting physics which is always a losing battle.  I have looked into adding wire to the ends but I really don't have room for that. 
I MAY be able to fit a 102' G5RV if I run the antenna at a diagonal to my house running it from a lower corner of the roof, up to the mast top, and then down to the back of the garage roof, it would be an inverted V as well.
The mast location is such that the feedpoint would no longer be at the top of the V, creating a dog-leg in one leg of the antenna.
I am not sure how great an antenna this would be but perhaps I could at least load it up on 75m!

Opinions?

Thanks again.

Rich

[WV Hoopie]

Rich,

Looks as though finding an antenna that fits into your lot, is a problem. I've got the same issue, a 40 meter dipole fits, just not enough land for a 75 meter dipole. So the fix was a 75 meter full wave loop. Some of it is about 25 feet in the air, some of it is maybe 5 feet above the dirt. No feed line per say, the ends connect to feed thru insulators on one side of the shack. The other end of the feed thru insulators (inside the shack) connect to the tuner with a short piece of open wire. Use anything on the house or lot to get the wire up in the air, keep it as close to a square shape as possible. Good chance you can pace off the distance walking around the lot to get as close to 1005/frequency. Don't worry about where you connect feed line to the loop. They are forgiving! My Johnson Matchbox will load my 75 meter loop on 40 meters as well which surprised me. Matchboxes are not known to have a wide impedance range.

As the band opens this fall contacts along the west coast are easy. Afternoons 3 PM the 3.870 gang is on and once again, contacts are a piece of cake with Oregon and Washington stations.

Don't limit your self by just considering one type of antenna.

73's
wd8kdg
craig

[Tom WA3KLR]

Rich,

I never had a G5RV, but I wouldn't switch to one.  My advice is to stick with the center-fed dipole and ladder line of 60 - 80 feet length and build a tuner that does deal with the load impedance.  I think this would be the most efficient antenna system for you.

GL.