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Author Topic: 40 meters indoor antenna  (Read 1793 times)
KK4MRN
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Posts: 77




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« on: February 16, 2013, 09:48:01 PM »

What would be a good antenna for 40 meters indoors?

Since impedence matching is very important, do you think it would be best to get an antenna tuner for 40 meters or maybe a SWR meter or directional watt-meter? 

My initial radio I want to use it with is an HT that can receive 40m SSB/CW/AM -- Kenwood TH-F6A.   

I will be re-using this antenna for other radios down the road, such as, home built SSB/CW receiver for 40m.  But eventually, I will buy a transceiver for 40m.
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K2DC
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Posts: 1340


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« Reply #1 on: February 17, 2013, 03:39:12 AM »

For the initial radio, impedance matching won't be critical at all since it's only for receive.  Since you don't have a radio that transmits (unless you have an impedance analyzer), you probably won't be able to closely match it anyway, so just us a calculated length.

What you can put up depends on where in the house it will go.  I would suggest a 40M dipole fed with coax, starting with 33" on each leg.  If it's going in the attic, just stretch it out as straight as you can.  If it's going in a room, I would put it at the juction of the walls and ceiling.  It won't matter much if you have to wrap the ends around the corners of the room.

Later when you have an analyzer, or a transmitting rig and SWR meter, you can trim the antenna for best SWR in the band portion of interest.  Performance may not be the greatest, as any indoor antenna is a compromise.  But any antenna is better than no antenna.

73,

Don, K2DC
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W5WSS
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Posts: 1605




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« Reply #2 on: February 17, 2013, 05:04:08 AM »

I built some indoor antennas. And am using one now.

Here are some that I used over a period of 12 years, though an attic was not available.

The results were reasonable for the circumstances and I progressed learning about the variables.

A multi band Doublet, Inverted V, Quadrant.
A vertically oriented vertically radiating rectangular loop
A horizontally oriented horizontally radiating rectangular loop.
A multi band vertical/W tuned horizontal wire radial sets of two per band routed around the baseboard.
A mono band 10m quarter wave ground plane with horizontal quarter wave radials.

The height above ground in every case about 18 ft.

All of the antennas served to make good contacts amidst many variables too complex to predict from the construction materials to any conductors around it.

Bottom line is to build and use them see what can be done 73
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W5DXP
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Posts: 3532


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« Reply #3 on: February 17, 2013, 07:34:34 AM »

What would be a good antenna for 40 meters indoors?

If you like to experiment, here is an experimental room antenna for 40m based on EZNEC.

http://www.w5dxp.com/40mRoom.JPG

I haven't tried it but the EZNEC file is available from:

http://www.w5dxp.com/40mroom.zip
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 12986




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« Reply #4 on: February 17, 2013, 09:39:44 AM »

For just receiving, impedance matching isn't important.  I've just used 10 or
20' of light hookup wire draped over a bookcase.  In fact, with my VR-500
I had to keep the wire short enough that it wouldn't overload the receiver.
So don't over-think the problem to get started.

The effectiveness of any antenna (for both transmitting and receiving) will
depend on the construction of your building:  concrete reinforced with steel
mesh is going to shield you from most signals no matter what type of indoor
antenna you use. Stucco has a layer of chicken wire mesh inside, but that
is generally more of a problem on VHF.  Wood is relatively transparent to
RF.  If you have ducting and/or electric heating in the walls or ceilings,
that can affect the antenna performance as well.  That's not to say that
you won't be able to receive signals - you'll just have to try it and see
how it works.  In addition, your height above ground will affect the
radiation patterns.


For an indoor transmitting, something like W5DXP's "room antenna" is a
good approach, though you can't count on an EZNEC model to include all
the wiring, ducting, etc. in the vicinity.  I've designed some similar antennas
using folded driven elements to raise the impedance.  In the end you'll
have to see what best fits the space you have available and adjust it
accordingly.

One thing to remember is that you can drill small holes through the tops of
the walls to run the wire from one room to the next, easily covered with
spackle when you move out.  You may find that you can squeeze a 40m
full wave loop running around through several rooms, or a 40m dipole running
down the hall, or wherever you have room for it.

You don't need fat wire (though it may help with some compact versions).
Magnet wire, or thin stranded hookup wire with an insulation color that
matches your walls,  can be put up with pushpins around the ceiling or
walls.  In one apartment I used thread (or dental floss) to tie the corners
of the wires to the pushpins.

A "magnetic loop" is another possibility, though more of a construction
project.


In every case where I've used indoor antennas, putting a discrete wire up
on the roof has worked better, but that isn't always practical.
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K7KBN
Member

Posts: 2754




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« Reply #5 on: February 17, 2013, 10:00:09 AM »

For the initial radio, impedance matching won't be critical at all since it's only for receive.  Since you don't have a radio that transmits (unless you have an impedance analyzer), you probably won't be able to closely match it anyway, so just us a calculated length.

What you can put up depends on where in the house it will go.  I would suggest a 40M dipole fed with coax, starting with 33" on each leg.  If it's going in the attic, just stretch it out as straight as you can.  If it's going in a room, I would put it at the juction of the walls and ceiling.  It won't matter much if you have to wrap the ends around the corners of the room.

Later when you have an analyzer, or a transmitting rig and SWR meter, you can trim the antenna for best SWR in the band portion of interest.  Performance may not be the greatest, as any indoor antenna is a compromise.  But any antenna is better than no antenna.
73,
Don, K2DC

That's going to be an awfully short 40m dipole, Don.  Or did you mean 33' on each leg?
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73
Pat K7KBN
CWO4 USNR Ret.
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