A question from an antenna ignoramus

[Edward Cain]

Hi Guys,
   I will be building a ham shack (wood construction shed, 12'x18') in my back yard. My antenna will be an 80M dipole strung between two trees approx. 50" high at endpoints and no support in between. The shack will need to be located somewhere under the span of the dipole.
   My question is: Is there any preferential location other than economy of feed line length? E.g. tuned? open wire/ladder line feed length vs. coax requirements.

   If if matters, I will be using various tx's including T-368, Viking 500, V-II, K3 and have available antenna tuners such as KW matchbox.

   Oh, and any other advice about construction of shack would be appreciated.

Thanks in advance,
Ed KJ4JST

[Steve - K4HX]

Optimally, you would want the shack at the center of the run of wire for your 80M dipole and located directly under the wire.

[Edward Cain]

Thank you, Steve, for the quick reply. Can you elaborate on the reasoning?

[Steve - K4HX]

It's best to have your feedline at right angles to the dipole. It also makes for the shortest run of feedline.

[W7TFO]

What Steve said, and that 80 (75)m dipole can be fed with a piece of 50-Ohm coax directly, with just a small strain insulator at the top.

-------(OO)-------
         X     Y

OO=insulator
X=center conductor
Y=braid


No tuner needed if you cut the horizontal for your planned op freq.

73DG

[KB2WIG]

Small refrigerator, microwave oven, Air conditioning, heat, and a pot to


klc

[W7TFO]

Heat?

Ain't that what transmitters are for?

73DG

[Edward Cain]

Thanks 'TFO and Steve for the info. And 'WIG, definitely on the refrig and air cond. In winter, I'll try to get by with heat from TX's. And, at least at night, I'll wiz out the window

Ed

[KB3DKS]

  I don't know how others would feel about this, but with your proximity to the antenna and high power capability, since it is new construction, I would screen the entire building including the windows. Probably chicken wire?
 Will keep the T-3 out, computer birdies in, and make egress difficult for any burglars.
 Tied to the station ground.
 At least a 30A minimum 220v feed. 60A preferred.

Bill

[Edward Cain]

Bill,
   Never thought about shielding. Good advice.

Ed

[flintstone mop]

RFI would be big problem under the antenna.....maybe tin roof as some sort of farady shield?? Nothing worse than to de-bug RFI in new construction. 80M and higher starts to be a problem. I read you have a T-3??? Nice TX and hi-power....more reason to keep the shack away from the antenna.
Design the feedline (coax) to come straight down and bury it to your shack which should be,,,IF YOU HAVE THE PROPERTY,,,100 feet away from the antenna.
What band or bands will be working?? Antenna only for 80M??
Consider something to get you to 160M, 50 feet high is respectable, open ladder line (6inch space NOT 450 window line) and a real good balanced tuner. Very low loss even with a 10:1 SWR on the OWL. My ladder line is not perfectly straight perpendicular down but at an angle and no problem with hot signals on 160M.
There is a HUGE tuner for sale in an Estate Sale in this forum, OR build the famous K1JJ tuner...And Palstar makes a nice heavy duty truly balanced tuner. BT1500A
You will get many thoughts here. Consider everything.
Fred

[KL7OF]

12' X 18' will never be enough space

[ab3al]

looks like everything is covered here but if your using a big johnson why not feed it with open wire line.. would be lighter giving the antenna maximum height at the feed and would make it a lot easier to multiband it.  on 75 the coax would be great but if you check the cebik pages it looks like 50-60  ft of co-ass would yeild about 6 db of loss not to mention very high voltages at that power level on other bands due to the miss matched antenna. this is not good if you are panning on using 8x for the light weight. not to mention that since its a balanced antenna the matchbox would take care of that part without hanging at least 2 lbs of balun  (commercial or coax) at the feed.  matter of fact you may want to plan a lexan plate about 1` by 1` on multiple sides of the shack for a feed through incase your antenna farm grows in other directions.. above eye level inside and out is a good place.

just my 1.5 cents.. damn this economy stinks

[K3YA]

Most of my HF antennas have been in close proximity to the shack, as yours will be.  In this case getting RF into the audio chain can be a significant issue.  This is especially true with the power level that you are planing.  I have found that a balanced, open wire fed antenna and a link coupled tuner can be more resistant to RF getting back into the shack. 

Usually I'll let the antenna coupler "float" with no direct ground connection But I also will try grounding the coupler's tuning capacitor frame and see if that provides better RF isolation of the audio gear.  If the antenna is left floating a spark gap for static/lightening discharge should be provided.

When I used coax feed in some cases I added a coil of coax where the feedline enters the shack in order to reduce RF into the audio gear.  I'm not sure if this is acts as a choke, or simply changes the length of the feedline.  Some feedlne lengths seem to be more prone to RF in the shack.

Check out the new "Mini-Split" systems for heating and cooling.  I thing one of these is going to be the next addition to my ham shack.

[ke7trp]

I am not sure where you live but if you are any where near a port of entry, a 20ft or even 40ft ocean container can be made into a great ham shack. Out here in AZ, They are $1200 to $2000 delievered. Solid Metal, RF proof, water proof ect. You can put some windows in, some cheap carpet, ac/heat and your good to go.

C

[Steve - K4HX]

The sea container would provide good shielding. Another option would be one of the metal military shelters. You may need to put several together to get the needed size.

For standard construction, just use foil-backed drywall and tape the seams with aluminum tape. You'll have plenty if shielding.

[Edward Cain]

   Thank you to all who have replied. I have another option which is to move my workshop to the shed and keep my radios in the basement. This will increase the distance to the antenna somewhat. Maybe 120 ft. Is this marginal increase going to make much of a difference in terms rf in the shack? If not, I'm probably better off in the shed where I can build in some rf shielding during construction.
   I don't think my wife would look favorably upon a metal shipping container in the backyard, unfortunately. But, otherwise, it's a cool idea.
   I'm not familiar with metal military shelters. Are their aesthetics similar to a shipping container?

[K3ZS]

The sea container would provide good shielding. Another option would be one of the metal military shelters. You may need to put several together to get the needed size.

For standard construction, just use foil-backed drywall and tape the seams with aluminum tape. You'll have plenty if shielding.

Also foil backed fiberglass insulation works good, my house has that and not much can be received inside.

[KB2WIG]

"  Ain't that what transmitters are for? "

Some use transmitters that are opperated with letters higher than "C". I would be remiss if I left them out.


klc

[The Slab Bacon]

     I don't think my wife would look favorably upon a metal shipping container in the backyard, unfortunately. But, otherwise, it's a cool idea.
   I'm not familiar with metal military shelters. Are their aesthetics similar to a shipping container?

Some cheap vynal siding attached with self-drilling TEK screws or some cheap T1-11 wood siding attached the same way and you're good to go. Water tight, burglar resistant, you're good to go. Just cut a hole in the side for a window A/C and add a "keggerator" and you got the world by the BA's........................

[WA1GFZ]

Polyisocyanurate foam insulation with foil on both sides works well with aluminum tape ofer the joints. I have to stand next to the door to get a good cell connection.
A place I worked we used copper screen and soldered the joints.
The johnson match box may need the feeder length adjusted to get a match on all bands. Also would work on 160 if fed as a top loaded t.