Stealth Antenna, No Ground. How's My Plan?

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RadioDaze

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Please tell me if I've misunderstood or overlooked anything...

I need to make a really stealthy, easy-up, easy-down antenna. Very strict condo. I’ll put it up when I need it, particularly at night, and I want something that doesn’t rely on a ground plane or counter-poise. I figure a 1/2 wave to 5/8 wave antenna will solve that, so I’m considering the Diamond NR73 models Diamond® Antenna ~ NR73BNMO Dualband Mobile Antenna or NR770 models Diamond® Antenna ~ NR770HBNMO Dualband Mobile Antenna which are half wave at VHF and 5/8 at UHF. I would just stick it on a mount at the end of a pole, either PVC or (more likely) aluminum. Just 6 or 7 feet, maybe less, to clear the rain gutter. I’d use about 25 ft. of LMR 240 Ultra.

Does it sound like I’m on the right track for a non-grounded antenna?

My primary frequency target is centered right around 495MHz, 20 MHz width, for LAPD, P25, about 30 miles to the northwest. Half of their freqs are around 484 MHz, the other half around 506 MHz. I can get some of it indoors with a rubber duck, with a lot of bad decoding on many channels, and with a mag mount on the balcony rail, I do a little better. The roof covers my balcony to a point about 10 inches past the rail, so I’ll make a bracket to hold the pole out a foot or so past the rail. I just want to get the base of the antenna above the gutter. I’m on the top floor of a 3-story building. The roof slopes upward, in the direction I want to receive, to a point about 7 or 8 feet higher that the gutter, and there’s no hope of getting above that. Stealth is key, and I want this thing noticed as infrequently as possible. There is MUCH better reception of my target freq on the other side of the building.

Other comments: It would be nice to be able to TX at 2 meters and 440, hence the dual-band antenna.
My 800 MHz reception is already good - I could use a paper clip to pick up my local TRS. This thing should be lightweight. Attic antenna: no way - the roof has metal flashing overall. Sealed like a can of Spam. I did consider a small yagi, but I think I'll be too close to the roof.

EDIT - seeing a few bad reviews on the Diamond, maybe this Maldol would be better: http://www.eham.net/reviews/detail/4378
I'm wide open to suggestions.
 
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prcguy

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In my experience ham band antennas really drop off in performance outside the ham band and at 500MHz you will probably be very disappointed. Do you have an attic?
prcguy

Please tell me if I've misunderstood or overlooked anything...

I need to make a really stealthy, easy-up, easy-down antenna. Very strict condo. I’ll put it up when I need it, particularly at night, and I want something that doesn’t rely on a ground plane or counter-poise. I figure a 1/2 wave to 5/8 wave antenna will solve that, so I’m considering the Diamond NR73 models Diamond® Antenna ~ NR73BNMO Dualband Mobile Antenna or NR770 models Diamond® Antenna ~ NR770HBNMO Dualband Mobile Antenna which are half wave at VHF and 5/8 at UHF. I would just stick it on a mount at the end of a pole, either PVC or (more likely) aluminum. Just 6 or 7 feet, maybe less, to clear the rain gutter. I’d use about 25 ft. of LMR 240 Ultra.

Does it sound like I’m on the right track for a non-grounded antenna?

My primary frequency target is centered right around 495MHz, 20 MHz width, for LAPD, P25, about 30 miles to the northwest. Half of their freqs are around 484 MHz, the other half around 506 MHz. I can get some of it indoors with a rubber duck, with a lot of bad decoding on many channels, and with a mag mount on the balcony rail, I do a little better. The roof covers my balcony to a point about 10 inches past the rail, so I’ll make a bracket to hold the pole out a foot or so past the rail. I just want to get the base of the antenna above the gutter. I’m on the top floor of a 3-story building. The roof slopes upward, in the direction I want to receive, to a point about 7 or 8 feet higher that the gutter, and there’s no hope of getting above that. Stealth is key, and I want this thing noticed as infrequently as possible. There is MUCH better reception of my target freq on the other side of the building.

Other comments: It would be nice to be able to TX at 2 meters and 440, hence the dual-band antenna.
My 800 MHz reception is already good - I could use a paper clip to pick up my local TRS. This thing should be lightweight. Attic antenna: no way - the roof has metal flashing overall. Sealed like a can of Spam. I did consider a small yagi, but I think I'll be too close to the roof.

EDIT - seeing a few bad reviews on the Diamond, maybe this Maldol would be better: Maldol EX-107/EX-107B Product Reviews
I'm wide open to suggestions.
 

RadioDaze

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Roof is sealed under flashing, so no signal is going to get through. A vertical monitor antenna is fine with me, if it doesn't need ground radial or other stuff hanging off.
 

hertzian

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I'm with prcguy on this about the dual-band ham antennas at 500 mhz. I also like doing the stealth bit. :)

Perhaps make a very thin simple vertical dipole for near 500 mhz and see how that works on the balcony. If you cut 5-1/2 inches of wire for each dipole element (one to the center conductor and the other to the braid), and hang it vertically with the coax coming out horizontally for at least 6 inches, how does that work?

You might be able to hide this pretty well....
 
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N_Jay

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Roof is sealed under flashing, so no signal is going to get through. A vertical monitor antenna is fine with me, if it doesn't need ground radial or other stuff hanging off.
What kind of roof is it?

Unless it is metal it probably only has 3 to 12 dB of loss. Compromised antennas are often worse.
 

RadioDaze

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What kind of roof is it?
It's some kind of composite molded tile, not really sure. But the whole roof is metal-lined underneath the tiles. I don't really have an attic per se, since the ceilings are vaulted on the top floor. I've raised an antenna up toward the top of my ceiling with no decent results. I'm in a huge building holding about 45 condo units. The bulk of the building lies in the direction I want to receive. So that's a problem, but not as much as the sealed roof.

Perhaps make a very thin simple vertical dipole for near 500 mhz and see how that works on the balcony. If you cut 5-1/2 inches of wire for each dipole element (one to the center conductor and the other to the braid), and hang it vertically with the coax coming out horizontally for at least 6 inches, how does that work?
Seems like it would be easy enough to test. If I use really thin components, esp. the horizontal portion, it might pass, stealthwise. For one thing, it would tell me if getting any antenna out above the gutter would even help at all, like a proof of concept test. If that tuned dipole didn't help, then I should just forget the whole thing.

I could stick a more conventional mobile scanner antenna at the end of a pole, but is there one that would be forgiving about the ground? Or if not, could I use an RF choke a little ways down on the coax to turn the shield into somewhat of a counterpoise?

Or is there a decent single-element vertical wide-band monitor BASE antenna out there?
 

riccom

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I remember looking a while back about a vent antenna, one that fits over the vet on the roof!

I may have to find it, but i heard good things about it, it looks like a extension of the pluming vent and can be painted to match the house or the vent its self!

give me a few and i will find it
 

hertzian

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RadioDaze[/quote said:
I could stick a more conventional mobile scanner antenna at the end of a pole, but is there one that would be forgiving about the ground? Or if not, could I use an RF choke a little ways down on the coax to turn the shield into somewhat of a counterpoise?
Despite all the advice and software modeling, in this instance stealth might mean just doing it empiracally to see what just gives you a better signal. It makes us textbook-types shiver, but sometimes you just do what you have to do.

One thing I'd avoid is any use of a 5/8th wave design since that relies upon a very large ground plane to work properly. Without a large ground plane, the elevation angle rises quite a bit.

Ferrites are out since they don't usually work well beyond 300 mhz or so, and if they did, you'd need quite a bit to do it right. I use type-43 ferrites for exactly this purpose on VHF, but still we're talking about snapping on about 8 of those, 4 at the quarter wave point and 4 at the half-wave point for a vertical dipole using the braid as the lower element. That would be pretty visible even if ferrites worked up to 500 mhz.

You *might* be able to wind a small coaxial-coil choke, but then again, this might be visible.

If you can get away with a dipole, I suppose you could put a TEE on the end of the vertical support and use 6 inches of pvc for the horizontal section to support the feedline, but even this might arouse suspicion. It would however serve as a reliable reference.

In this case, I'd consider a UHF j-pole - I've seen them made from twinlead, and in fact at HRO I've seen custom dual-band jobs that don't appear in the catalog. Then again, these are designed for the amateur bands but might be the compromise you would be willing to live with. Thing is, they are build out of twinlead, so you'd have to support them somehow. Placing them inside pvc would lower the resonant frequency, so if you are planning to just temporarily erect the support when needed, it might last a bit longer than expected if you used a non-metallic stick attached to the mast to hang the top of the twinlead dual-band j-pole from.

Worst case, you could just attach 5.5 inches of wire to the center element and then raise that up and see how well a quarter-wave spike with no radials work. Not ideal for sure, but if it is just a matter of clearing obstacles to get some quieting, damn the textbooks! :)

Note that "folding back" the shield is innefective, as a shield used in this folded back way has no air-gap like a real sleeve, so we are really fooling ourselves here and have basically just the quarter-wave stick in the example paragraph above. :)

If it were me, I'd just try the quarter wave stick first. Then as a compromise for your operating purposes, the dual-band amateur twinlead j-pole. OR, perhaps just barell a handheld duck up there, but don't count on anything that uses 5/8ths to work well.
 

RadioDaze

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I remember looking a while back about a vent antenna, one that fits over the vet on the roof!
I had a Ventenna on my last condo about 15 years ago and loved it. But I had roof access and attic access to bring in the lead. I already considered that for this condo, but I'd have to do some complicated stuff to bring in a lead, like fishing it in along the A/C freon tubes.
 

hoser147

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Im wondering the same thing hertzian. No ground plane for the mobile antenna. Also as far as a stand one OP had his antenna on a Mic Stand, there are alot of good pictures in the Shack Photo section. Good Luck
 

RadioDaze

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One thing I'd avoid is any use of a 5/8th wave design since that relies upon a very large ground plane to work properly.
I guess it would be a 1/2 wave that wouldn't need a GP.

Ferrites are out since they don't usually work well beyond 300 mhz or so
I had forgotten about that

I suppose you could put a TEE on the end of the vertical support and use 6 inches of pvc for the horizontal section to support the feedline, but even this might arouse suspicion. It would however serve as a reliable reference.
Since the dipole itself would be short, maybe the overall size including the horiz. section wouldn't be bad, especially at night. And "paint it black" as Mick Jagger would say.

...I'd consider a UHF j-pole - I've seen them made from twinlead... ...Placing them inside pvc would lower the resonant frequency...
Is the PVC pretty radiopaque? I figure it must have some amount of lead in it, courtesy the vinyl.

...see how well a quarter-wave spike with no radials work. ...perhaps just a handheld duck up there...
Not sure if these would even overcome the feedline losses. I did clamp a rubber duck to the rain gutter in the past, and it wasn't very effective.

A j-pole could be custom-cut at least, so I could maximize the ~500 MHz focus, and still have something working in the VHF range.

But I really like your dipole idea as a test, since it will tell me if I'm barking up the right tree. I'll just use a piece of wood for the model, maybe with some 1/4" aluminum elements for a little wider bandwidth. Painted black, and stuck to the end of some wooden closet pole, it could be good enough. Although I'd like something a little more moisture proof than that for a final design.
 

rankin39

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Rather than just a quarter wave piece of coax center conductor, I'd slide the outer conductor down around the outside of the coax, tape it in place and make what they used to call a "coaxial antenna". This is technically a ground plane with the radials pulled down to a vertical position along the coax to make the whole thing skinny. I'd try that first since it only requires the run of coax and some electrical tape. If that doesn't get you the reception you want, I'd try the uhf J-pole element. If you can manage the space and visibility, a uhf TV corner reflector should do the job nicely. Tell the management you're having a really hard time getting digital HD TV broadcasts and just want to stick the corner reflector up at night. ;-)

Bob, WoNXN
 

RadioDaze

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Rather than just a quarter wave piece of coax center conductor, I'd slide the outer conductor down around the outside of the coax, tape it in place and make what they used to call a "coaxial antenna". This is technically a ground plane with the radials pulled down to a vertical position along the coax to make the whole thing skinny. I'd try that first since it only requires the run of coax and some electrical tape.
Bob, WoNXN
Thanks Bob. Is this going to be functionally the same as a center-fed dipole, or are there some relative disadvantages? The big plus of course would be its thin profile.
 

hertzian

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The biggest disadvantage to the braid folded back over technique is that it looks good on paper, but is not functioning as a sleeve - that takes a good dielectric such as air to work. The coax jacket is not a very good rf dielectric. To do it right, you'd need a tube with about 1 inch or more spacing between it and the coax.

Will it work? Somewhat, but we are fooling ourselves (myself included) into thinking that it is a real bottom element for a dipole, despite the apparent changes in swr if put it on a meter. Much of the initial enthusiasm comes from just having an antenna up and out in the clear. :)

In fact, in my HF days using antennas just like these, putting any appreciable power into it just arcs over at the very end of the folded braid to the feedline braid. To rf, since there is no appreciable dielectic spacing, it just couples right to the common mode (outer skin-depth of the feedline braid). When it did accept power, it did so with a pretty unusual elevation angle due to the rf coupling to the entire braid - so what I really had was a quarter wave spike and the rest of the coax as the other side of the dipole as far as rf was concerned.

I guess what I'm saying is that the folded-braid technique is about 99-percent similar to not doing any folding at all, and just trimming a quarter wave spike. The results are extremely similar.

PVC radomes - be careful here too. Aside from lowering the resonant frequency, it can also change the elevation angle - something to consider for even rf-friendly pvc.

I think you are on the right track by building just a simple dipole as a solid reference from which you can compare further experiments to.
 
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RadioDaze

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Just as well with the braid folding - for all my effort all I got was quarter inch of progress and a blister on my thumb. Old Tandy Mini 8 foam.

My dipole was coming along, but I had forgotten how difficult 1/4" solid aluminum was to solder, and I have a head cold that worsened and took away all my motivation to go to the hardware store for some copper lugs or the like. But maybe tomorrow...

What about a design that would steal from the bazooka... using a hollow tube like a piece of aluminum antenna mast of a certain length with an NMO on a drilled-through metal cap on top. Then I could put a frequency-specific whip on it. That would get me closer to the 1" dialectric, without the unstealthy radials. I could place it on top of a section of PVC so that it doesn't try to couple with anything else. Kind of a Shakespeare Small Stick.
 

RadioDaze

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Progress report:

I improvised and got the dipole working. Placed it in same spot as my "control group" Comtelco scanner antenna on mag mount on balcony rail. Results: 2 S-units better in target band than Comtelco, and better P25 decoding on LAPD. Looking good. Also picking up San Diego 800MHz TRS many miles away, line-of-sight. So local 800 MHz should be fine. And I'm using crummy coax to boot.

Next is a test at height above rain gutter. Painted black, this thing might be just the ticket. I can rebuild the horizontal stand-off in a smaller form factor instead of chunky wood.

Still wondering about the bazooka idea mentioned in my previous post.

Or maybe a vertical dual-band K-shaped fan dipole, so I can pick up better VHF...

Ah, the slippery slope to a discone... and sanctions by the HOA.
 

hertzian

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Hm - that link isn't coming up right now, but I know what you are thinking.

Before you get to that stage, the first thing I'd do now that you have met one objective, is to ditch the crappy RS coax, and use that LMR you mentioned. Take notes on the s-meter readings etc, AND do the same test with the attenuator active!

With the attenuator temporarily active, you can gather data that might make it easier to determine if future antennas are working better than a plain dipole especially if the target signals are full-quieting.

While you've still got the soldering iron out, how about cutting a dipole for 165 mhz? About 17-inches on a side for the dipole elements. This will get you down into the VHF range.

BUT, now you can see how well this antenna will work at 160 mhz natively, and at 495 mhz +/- at the third harmonic! Thing is, at the third harmonic, you will have a cloverleaf antenna pattern. BUT this is in a perfect world, and in the real world, this elevation angle may not be as bad as calculated what with the antenna being close to all that roofing material.

Now we're getting into compromises, but I think it would be worth a shot before spending $$ on prebuilt antennas for rx-only at least.

A lot of stuff is going to be empirical - off the top of my head maybe think about using a RS "sputnik" ground plane with the radials removed and painted black with rf-friendly paint? Here we go.... :)

At the very least, swap out the coax with what you plan to use permanently, and maybe try the 165 mhz dipole and see how it does on vhf and uhf at the third harmonic in your environment.
 

prcguy

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I keep seeing a rain gutter mentioned, is it metal? How bout an NMO mobile mount on a small right angle bracket just inside the gutter and use the metal gutter or a few radials inside a plastic gutter. You would only have a whip with loading coil sticking up from that point. You could then try any NMO antenna like a Larsen tri-band or there is a larger Larsen VHF/UHF dual band available that is tuned for commercial frequencies.
prcguy
 
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