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Author Topic: Mobile screwdriver antenna on condo roof  (Read 9068 times)
NEWHAM77
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Posts: 12




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« on: September 23, 2011, 11:13:06 PM »

Hello all. I will be buying a condo with the usual HOA rules about antennas. Just for the heck of it I suggested that I might withdraw my offer because of my hobby that requires a external antenna. The real estate sales guy asked for a example of what I would require, so
I printed out a internet photo of a Scorpion screwdriver 680 with the black widow color.
The HOA approved it ! So now I can mount it on the roof just above a very small crawl space in the attic. My question is will it work with radials that are inside the crawl space ?
The HOA made it very clear the antenna had to match the photo i showed them. Thanks

I forgot to say why I picked that antenna....my real estate guy asked me to show him some black stuff. I asked him why and he told me that the roof had a lot of black vents
so I looked with him on the eham reviews and used this one...
« Last Edit: September 24, 2011, 12:05:30 AM by NEWHAM77 » Logged
K4KWH
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Posts: 51




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« Reply #1 on: September 24, 2011, 08:07:07 AM »

How u gonna deal with the counterpoise wires; you think the HOA is going to be OK with that?  There's more to it than sticking a whip on a roof and expecting it to work.  Just sayin'.......


J
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M6GOM
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Posts: 915




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« Reply #2 on: September 24, 2011, 12:37:13 PM »

How well it works depends on the material the roof is made of. If the roof is metal, you don't even need them - just use the roof.
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WA8FOZ
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Posts: 192




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« Reply #3 on: September 24, 2011, 08:33:50 PM »

Lay radials ON the roof, using fine wire with insulation of matching color. You might also consider carefully tucking them under the shingles. You could even make a whole mesh radial field that way if you wanted to try that.
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NEWHAM77
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Posts: 12




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« Reply #4 on: September 24, 2011, 11:50:46 PM »

Thank You for the comments so far. I do not know what the roof is made of yet. Getting approved for this screwdriver antenna on the roof is way better than most comments I
read about the HOA crap that most HOA ham's have to put up with. This is not perfect but
way better than running secret wires or using rain gutters in my opinion.

So, my question is can I have the counterpoise wires inside the crawl space which is directly
under the place the screwdriver antenna will be located...the crawl space is about 30 feet long
and only a couple feet high.  Also the antenna and the crawl space are directly above my little
shack in the garage.

Thank You !

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K7JQ
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Posts: 341




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« Reply #5 on: September 26, 2011, 10:39:54 AM »

Can I assume the roof is not metal, and the trusses below it are not metallic? If this is the case, then the radials must be of resonant length for each band desired (as little as two per band), and should be connected to the coax braid as close as possible to the bottom of the screwdriver...preferraly no more than 6" to 9". Only if the screwdriver is ground-mounted, the radials don't have to be resonant, as many as possible. In your case, I guess you'll have to drill a hole in the roof, somehow mount the antenna practically right on top of it, and fan out the radials in the crawl space directly beneath it. Sounds like a tough job with only two feet of vertical space to crawl around in, and with only 30 feet of horizontal space to work with, you'll be limited to resonant radials for 20 meters (approximately 15-16 feet on opposite directions of center) and up in frequency, assuming you can mount the antenna in the center of the 30 feet beneath it. I like the idea of laying the (resonant) radials on the roof with some way of camouflaging them. Should be much easier. Also, with the operating position so close to the antenna above it, you might encounter some RFI issues. If you can get it to work, you should be pleased with the results. I use a ground-mounted screwdriver in an HOA community, and have been very happy with its' performance. Good luck.

73,  Bob, K7JQ
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WX7G
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Posts: 6080




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« Reply #6 on: September 27, 2011, 11:16:37 AM »

You antenna will work well with radials installed in the crawl space under the antenna as long as the roof (and insulation) are not conductive.

Use two resonant radials per band run opposite one another if you have the room. If you don't have the room one radial will do the job.

Place ferrites on the coax at the antenna or use a current balun. This will cause most of the counterpoise current to take the path of the radials rather than the coaxial cable shield.
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W6RMK
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Posts: 651




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« Reply #7 on: September 29, 2011, 09:28:31 AM »

Don't obsess too much about whether the radials happen to be resonant.  (If for no other reason than the effect of the lumber and roof will be hard to predict).

You could also throw radials out on the surface of the roof as 'FOZ suggested (It's what I've done.. I'm tired of crawling through fiberglass insulation and hitting my head on roofing nails sticking through).  They can be pretty fine (and invisible) and just tucked under corners and edges.  AWG 20 or 26 would probably work just fine, although bigger is better, but you shouldn't be agonizing about how you're going to hide those 2/0 welding cables.

As with any sort of artificial ground plane, more radials, of any length, is better. 

The case where tuned radials are really important is when you don't have any sort of adjustable matching network (think of the 2m ground plane with 4 drooped radials).

If you think about it, the whip on top of the adjustable coil isn't resonant either.  That's the whole point. The screwdriver runs in and out until the *system* is reasonably well matched.

You could hook your screwdriver up horizontally, with a matching whip on either end, and it would find a match.


The comments about needing GOOD RF chokes are exactly on point.  Buy yourself a pile o' #31 mix cores.  5-6 turns of the coax through a 2.4" core does wonders.  You'll need to (separately) choke the DC control wires to the antenna and the coax, and it probably wouldn't be bad to choke at both ends.

Jim Brown (K9YC) has a great writeup on RFI/chokes, etc.
http://www.audiosystemsgroup.com/RFI-Ham.pdf

31 mix toroids are available in lots of places (Mouser, DigiKey, etc.), and for all I know, the local ham store, if you have one.  They're manufactured by FairRite.


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KB3HJK
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Posts: 97




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« Reply #8 on: September 29, 2011, 09:12:31 PM »

I like the idea of making your antenna a bargaining chip! Good thinking. I think more salesman will bite in this economy.

Commision and low profile antenna, or nothing! McDonalds again, pal? Wrong decision.
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KC7YE
Member

Posts: 97




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« Reply #9 on: October 02, 2011, 06:22:45 PM »

We rent a condo when we go to desert for winter. Use a High Sierra screwdriver antenna with fair results. I just put out one counterpoise either on carport roof or on the ground, depending on what unit we get. If spent more effort would be better, do more each winter. Any way is better then mounted on car, and a lot more comfortable.
KC7YE
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AA1BN
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Posts: 56




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« Reply #10 on: October 22, 2011, 04:42:23 PM »

I'm in Melbourne, Fl., in a frikkin @#$%^&* "deed restricted community".

Before the HOA fined me fifty bucks and made me take it down,
I ran the screwdriver on the roof without radials, and just a ground
lead from mast to electric/water ground.

I have Japan, China, Italy, Russia, Cuba, Alaska and most
everything stateside logged (all CW, however).

Although we are beginning a recall action to recall all 9 members
of the HOA, I've decided that a wire around the total perimeter
of the house might be a better choice. I miss working 160, and
a screwdriver just won't do it.. A wire loop and my tuner should
work better than a vertical...

We'll see...

73,

John


« Last Edit: October 22, 2011, 05:51:30 PM by AA1BN » Logged
ZENKI
Member

Posts: 938




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« Reply #11 on: October 24, 2011, 06:51:18 PM »

MFJ have a new remote antenna tuner that would be perfect for deed restricted lots. I dont know how good it is.

http://www.mfjenterprises.com/Product.php?productid=MFJ-998RT

Its a 1500 watt tuner.
It would be easy to hide this tuner and a random length of wire or  vertical length of wire
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K2DC
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Posts: 1365


WWW

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« Reply #12 on: October 25, 2011, 07:13:37 AM »

Agree very much with W6RMK's advice not to obsess too much about resonant radials, even if they might help a bit.  If you think about it, mobile screwdrivers are design for use on the roof, trunk, hood or bumper of a motor vehicle which offers a very limited ground plane or counterpoise on HF and they work well.

As long as the roofing material isn't conductive, put as many radials as you can wherever you can and attached as close to the feed point as you can.  And the ferrites or choke balun is also very good advice.  After all, one of the biggest secrets to HF mobile operation has always been to get all the RF outside the car.  Your case is no different.

GL & 73,

Don, K2DC
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K3RE
Member

Posts: 35




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« Reply #13 on: November 24, 2011, 06:10:42 PM »

I am planning on a ground mounted Tarheel model 100 about 18 ft from the rear of my house off the cornrt of my deck. I am going to purchase their radial ring that comes with 10, 9' radials and room to add additional radials. Total coax and control cable run will be about 45' to my equipment. The new Ameritron screwdriver controller (digital motor readout) looks like a great way to remotely tune the Tarheel. I want to run either the ALS-600 or AL811 amp. Should I expect any problems running 600 watts with the antenna as close to the house? Would a RF choke help?
« Last Edit: November 24, 2011, 06:18:35 PM by K3RE » Logged
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