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Author Topic: Carport loop idea - please critique  (Read 10019 times)

Posts: 5

« on: December 20, 2013, 04:37:56 AM »

Please excuse spelling and grammar. I'm typing on a tablet in a waiting room.

I can't do a vertical, don't have private yard space for a standard / full size antenna, and putting anything on he main house roof is not possible.

I think I can put a loop up under the roof joists of our two car carport. It is wood construction and has a flat rubber-ish roof. One length of gutter on the edge furthest away from the open parking area. There may be some metal flashing around the edge of the roof - at least a foot or two from the planned loop location on any side.

So far my plan involves putting a square loop on/under the joists.
I have the space to make the loop about 17-18 feet per side.
I think I can hide an sg-239 in the joists with the loop.
For wire, the 26awg poly stealth from HRO
I haven't thought much about the coax feed from the sg-239. I may need up to 100ft to get to where I can put a radio.

Wire mounting questions.
Do I need to worry about suspending it from insulators or can I drill small holes through the wood to thread it around?
If I need to insulate it, will 2-3"zip tie loops hanging from small eye bolts be good enough?

I have two cfl bulbs in a fixture that will be about dead center of the loop. Are they likely to interfere with any aspect of HF?


Posts: 2685


« Reply #1 on: December 20, 2013, 05:40:18 AM »

What bands are you intending to work?  What power levels?

The CFI bulbs may or may not be a problem - no way to tell until you try the antenna.

Coax Cable Signal Loss (Attenuation) in dB per 100ft*

Loss*    RG-174RG-58RG-8XRG-213RG-6RG-11RF-9914RF-9913
1MHz    1.9dB0.4dB0.5dB0.2dB0.2dB0.2dB0.3dB0.2dB
10MHz    3.3dB1.4dB1.0dB0.6dB0.6dB0.4dB0.5dB0.4dB
50MHz    6.6dB3.3dB2.5dB1.6dB1.4dB1.0dB1.1dB0.9dB
100MHz    8.9dB4.9dB3.6dB2.2dB2.0dB1.6dB1.5dB1.4dB
200MHz    11.9dB7.3dB5.4dB3.3dB2.8dB2.3dB2.0dB1.8dB
400MHz    17.3dB11.2dB7.9dB4.8dB4.3dB3.5dB2.9dB2.6dB
700MHz    26.0dB16.9dB11.0dB6.6dB5.6dB4.7dB3.8dB3.6dB
900MHz    27.9 B20.1dB12.6dB7.7dB6.0dB5.4dB4.9dB4.2dB
1GHz    32.0dB21.5dB13.5dB8.3dB6.1dB5.6dB5.3dB4.5dB
Imped    50ohm50ohm50ohm50ohm75ohm75ohm50ohm50ohm

* Note: Coax losses shown above are for 100 feet lengths. Loss is a length multiplier, so a 200 ft length would have twice the loss shown above and a 50 ft length would have half the loss. This multiplier factor is why you should keep cable installation lengths between radios and antennas as short as practical!

Posts: 331

« Reply #2 on: December 20, 2013, 10:38:37 AM »

Re: Coax Loss in tables is for matched load and can be much higher for SWR > 1:1.

Posts: 2685


« Reply #3 on: December 20, 2013, 12:16:31 PM »

Re: Coax Loss in tables is for matched load and can be much higher for SWR > 1:1.

"I think I can hide an sg-239 in the joists with the loop. "

He will have a 50ohm load in most cases so the table is relevant

Posts: 13015

« Reply #4 on: December 20, 2013, 12:56:02 PM »

Will it work?  Yes, to some extent.  How well is a different matter...

The loop is about a full wavelength on 20m, and will have maximum radiation straight
up.  The problem with that is that the ionosphere doesn't reflect such waves back to
Earth on 20m the way it does on 80m.  You'll still have some radiation and make
contacts, but it won't be as effective as, say, a 40m loop.  On 10m it shouldn't be
too bad.  On 40m it is 1/2 wavelength, which will present a high impedance to the
tuner - you might have a problem matching it, but it is difficult to know for sure
because the resonant frequencies of the loop will be affected by the building in
unpredictable ways.  (One 40m loop that we strung around a roof ended up at 5.9 MHz

On 80m it will be quite short - efficiency will be somewhat poor but you probably will
be able to make contacts.

But if that is the only antenna you can put up, likely it will work much better than
the same amount of wire coiled up on a spool in the garage.

I've taken a lot of liberties putting up wire antennas, including stapling it to the back
side of the fascia boards.  So far I haven't had any problems up to 100W, but it is
always worthwhile keeping an eye out for them.  Folding a piece of plastic cut from
a milk jug around the wire and stapling that to the board is certainly better, and
your tiewraps should work as well.  If you have to drill a hole and pass it through a
board it may be worthwhile slipping some extra insulation over the wire, or passing
a piece of Teflon tubing through the hole with the wire inside that.

When you fire up the antenna, let your tuner match it at low power, then slowly
crank up the power while watching for a sudden jump in SWR, which is a good
indication if something is arcing.  Of course, the tuner will try to retune the antenna
when that happens (one disadvantage of autotuners) but you should see some sort
of indication that the impedance has changed.  A visual inspection (and listening for
arcing noises) while the transmitter is in operation is a good idea as well.

Posts: 5

« Reply #5 on: December 20, 2013, 01:59:47 PM »

Additional info

Max power is exceptionally unlikely to exceed 100w
Intended bands are whatever HF I can get out of it

Hmmm... the vertical radiation pattern comment gave me an idea - I might be able to put up a vertical loop on a side of the carport. The dimensions will have to be different though. I'll get out there with a measuring tape and see what options I have. It could bring in other concerns though, as one side would have it shooting directly into our unit, and the other is aimed at a quite near neighbor.

Posts: 1644

« Reply #6 on: December 20, 2013, 07:58:26 PM »

I have a 46ft horizontal loop mounted indoors below the ceiling using Teflon clips.

The system is fully automatic.

The auto tuner along with a 1:1 current balun, on the output of the tuner located directly between the LDG 200 Pro tuner and antenna using a male to male UHF adaptor are all secured and mounted to a special mounting bracket positioned up to place the balun terminals where the loop can be properly connected.

The height above ground in this case is a low 20ft

The system works reasonably relative to radiating all of the power input from 20m and up.

On 20m This system works reasonably but at a very high angle, however the angle lowers progressively as the system is utilized on each of the higher bands 18 15.12 and 10 respectively. on 15 to 10m I enjoy reasonable longer range sky wave contacts.

I  agree with Dale WB6BYU in that one should use caution and make every effort to install the antenna about 3" with stand off parts that are OK for Rf I have always used #12-#14 insulated wire.

As to the vertical loop feed it on a vertical leg with the tuner at the feed point this allows normal runs of antenna feed line that is operated at maximum efficiency since the line is matched losses are low, and run the antenna feed line to the equipment.

Consider a balun between the tuner and the loop feed point If you have an analyzer determine the impedance range for all the bands, this will help you to decide if a 1:1 or other is the proper ratio.

The 1:1 current balun can be excellent if your tuner can handle the excursions and impedance variations.

The balun can help with noise ingress on RX and common mode displacement currents on TX relative to various and semi specific cases/ issues where antennas are close to the domicile.

Perhaps you can install both antennas one need not be overly concerned about interactions between the two and lots of fun to include an antenna switch for rapid a/b switching between the two for antenna comparisons and sky wave DX etc.


Posts: 7

« Reply #7 on: January 03, 2014, 04:33:52 AM »

Just curious, have you thought about using a mobile antenna mounted on the back of your car?  I did this and ran some RG 58 along with the wireless screwdriver antenna controller and left the car parked in the driveway.  HOA could do nothing about it.  Worked great.   Secret is to get a GOOD mobile antenna.

Good luck

Posts: 1

« Reply #8 on: January 03, 2014, 03:09:58 PM »

Hi there -
I live in a town-house here in England also with no possibility of an outside antenna. Instead I have a loop in the attic/loft which is roughly a full wavelength on 20 metres, fed via a balun of indeterminate ratio (I think it's 4:1) with a random length of RG58 to an MFJ-945 tuner. The ground/counterpoise is the house central-heating radiators!!

The interesting thing is that with the tuner I can get the loop to radiate on 40 metres to 6 metres - although goodness knows why and I shudder to think what the efficiency is! Having said that, though, in 2013 with very casual operating I worked 67 countries using a maximum of 100 watts SSB. Of those, 35 were QRP 5 watts or less - in fact, 19 of that 35 were worked with 500 milliwatts SSB. There were also 17 countries on 6 metres with the best being Italy with 500 milliwatts.

I guess what I'm trying to say is just put up whatever antenna you can - anything is better than nothing! Even if you are limited to QRP operation it's still possible to have fun and talk to people.

73 - Keith G0RQQ
Lincoln, England.

Posts: 1644

« Reply #9 on: January 05, 2014, 03:28:04 PM »

G0rqq hello, Try a good balun Brand of known specifications such as Balun Designs, DX Eng. etc. You may find a noticeable drop in common mode displacement current Noise but not skip signals.

Noise ingress at my location dramatically reduced with the choke 1:1 ratio current type Balun directly at the antenna feed point and coaxial cable from the tuner input and the tabletop equipment.

I understand that your system is tuning for radiated signal but do you know by what percentage? and by what percentage of field strength per band perhaps it is worth some investigating.

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