“Never do anything to attract enemy fire – It annoys those around you.”
Murphy’s Laws of Combat.
UPDATED 5/8/223 This section will deal with various means to covertly operate radio systems while going unobserved. Any number of situations may call for these measures but the antenna system must still work!
“You can’t have antennas here.” Challenge Accepted!
Like many of us, I live in a neighborhood where now only satellite TV antennas are automatically authorized. (The power of group pressure). However as an individual homeowner I was able to get approval when I first moved here since the original CC&R/HOA documents did not prohibit antennas. In any event, a brief presentation to the HOA Board resulted in approval. However I keep them very low profile to keep the neighbors happy anyway. No towers etc.
“There are no rules here. We are trying to accomplish something.” Thomas Edison.
Cover, concealment, camouflage – or hidden in plain sight.
See that 17 foot aluminum Grumman canoe (hint – mounted on those black plastic rack-mounts)?
I see a quarter wave horizontal mobile antenna for the 20 meter Ham band. It worked fine! (With a little help from impedance matching.) The fun part of antenna experimenting!
An obvious solution if you need concealment and have access to a space like an attic. I have several VHF ground plane and discone antennas in my attic. The wood structure is pretty transparent to RF energy. Signal strength from distant VHF repeaters does not seem to be affected even when the roof is wet. Here’s part of my setup:
This one is a DIY Floppy Discone I made with 1/4″ galvanized screening, suspended from the rafters with paracord. It is fed with RG-213/U 50 ohm coax. Design dimensions of this one allow it to cover approximately 75 – 500 mc with acceptable VSWR so it works well for receiving and transmitting over that range. Simple, cheap, effective, very stealthy.
There is also a commercial discone visible further back towards the vent. These are particularly effective driving scanners or other wide band receivers such as my APR-1 radar surveillance receiver. There are also some wire ground plane antennas for discrete frequencies stapled to the rafters elsewhere.
Spaces like these also work well for suspended dipoles for lower frequency operations. I have also successfully installed fixed-aimed VHF/UHF yagis for reception of TV and FM broadcast signals.
Rain Gutter Antennas:
Below is a photo of a simple, effective and entirely covert HF antenna. It is the rain gutter along the front of a house. It is 80 feet long and off-center fed as a Windom with a single wire lead-in to a tuner. A GRC-109 will drive it directly without the tuner on 80, 40 and 30 meters and it has yielded good results within about 1000 miles at the 10 watt level.
At 10 feet above ground and operated “against” the house wiring/neutral radio ground, it makes for an effective NVIS-type antenna depending upon the freq/time of day. A simple solder lug under a sheet metal screw and then painted over makes the connection. The screw point penetrating the sheet metal is covered with silicone RTV to stop corrosion on that end. This antenna has been in continuous use for the past 15 years. Invisible.
My new gutters are the modern type, formed from a continuous roll of flat sheet aluminum – no seams. Seams? No problem. Sheet metal screws and RTV protection fixes that. Plastic? Just run some insulated wire above, inside or behind them. Simple works.
The covered wood rafters provide adequate “RF insulation”, even when the overlaying roofing itself is wet – I see minimal change in the SWR / complex impedance and any power loss appears to be minimal, certainly in terms of distant field strength “S Units”.
The down spouts, especially at ground level can also part of the system so make sure there can be no RF voltage or possible personal contact there. If there is concern, inserting a short PVC section between the gutter and metal downspout will isolate it. The $2.00 fix; cut to size. Alternatively, make the downspout itself from PVC and run an insulated wire up inside it.
Hardware stores have a wide variety of standard-size PVC downspout stuff. A simple straight section worked here. Good idea anyway, apply Common Sense. Think it through. SAFETY FIRST!
Gutter antenna connection “detail”. See the connection? Simple, effective.
Above: This gutter antenna is connected via an upper roof-level downspout where no personal contact is possible. This all-aluminum gutter is fed with coax with an unterminated shield (at this end, shield connected to radio ground at the other) to reduce nearby computer hash from inside the house. Experiment.
It is a quiet HF antenna that is untuned but it happens to be resonant in the 60 meter band. With a simple TEE “antenna tuner” the transmitter will feed it power and radiate it anywhere on HF. Produces contacts via NVIS for regional comms and DX out to several thousand miles with a 100 watt transmitter.
It works great and seems to be not affected by rain/wet conditions. Pretty invisible. After you are done testing, apply a blob of RTV to weather proof the connection.
Above: Another HF rain gutter antenna connection. Also an effective HF performer; this one is used primarily as a shortwave receive antenna. These aluminum gutters are made of 0.032 inch thick material and have an equivalent cross sectional area about the same as size 00 AWG wire. This produces a rugged, high conductivity antenna although not quite as conductive as copper of course. Pretty close. Hint: Big Yagi’s are made of aluminum.
These elevated, isolated gutter antennas do not produce an RF “shock” hazard to someone at ground level since their downspouts do not go all the way to ground level. They spill onto an intermediate roof area. At the low power levels I run on the 80-60-40-30 meter bands they are also RF safe to the building inhabitants.
However since they are in close proximity to the building (inhabitants) make sure you perform your MPE calculations. SAFETY FIRST!
Below is another HF gutter antenna with a slightly different feed connection at the end of the galvanized steel gutter (antenna). Solder lug is under a sheet metal screw, screwed into the gutter. The lead-in is concealed under the brown flashing strip. The screw/lug is protected by a blob of clear RTV weatherproofing. (Yup, the gutter needs to be repainted..) This antenna has also been in use for over 15 years. I have had no problems with galvanic corrosion or RF rectification but it is something to be mindful of. Simple, effective, invisible.
No gutter access? Just use the aluminum flashing strips as available.
Disclaimer: The NEC Code addresses antennas so follow your local jurisdiction requirements in all cases. Be aware. The ideas presented here are for informational purposes only. Safety First.
Below is another simple type of covert receive-only antenna. This time, it’s a 1/4 wave ground plane cut for the local airport tower frequency. It is simply an SO-239 with brazing rod soldered to the center conductor for the vertical whip and 4 radials sloping downward at 45 degrees. That is slipped over a 2 foot piece of 1 inch PVC pipe which conceals and protects the PL-259/SO-239 connection, the internal preamplifier and also provides a mounting capability. The PVC pipe is painted flat olive-drab and is zip-tied to the branches about 30 feet up in the tree.
I am currently using this antenna to drive my AN/APR-1 Radar Surveillance receiver listening to aircraft in the VHF/UHF AM bands. That receiver has inherent low sensitivity so this little preamp helps considerably. Some details here: http://www.n6cc.com/an-apr-1-apr-4-radar-comm-surveillance-receiver
The antenna includes an old Radio Shack TV/FM antenna preamplifier (appx 10 db gain, 5-900 MHz supposedly) and protected inside the PVC mounting mast. A notched PVC pipe cap keeps it somewhat weather proof. It drives some RG-6/U 75 ohm “TV” coax that I had on hand, down to the bias tee and receiver. It works very well in the VHF and UHF military AM aircraft bands. It drives my AN/APR-1 Radar/ELINT intercept receiver quite well.
It is very effective and entirely invisible from ground level. The proximity to the branches has no apparent ill effect and the antenna itself is positioned away from the trunk and actual foliage near the support branches. No apparent impedance change or performance loss while wet although at VHF frequencies I am sure there is some loss. I don’t notice it. That 30 foot elevation sure helps with line-of sight radio range though.
The PVC pipe is barely visible, even zoomed up close as in this photo. The antenna vertical element is made from copper alloy brazing rod (from “Harbor Fright”) and the radials are 14 ga copper wire. Sprayed with flat O.D. paint it is all but invisible.
This one is mounted 1/4 wavelength from the main tree trunk. I stapled a vertical, 1/4 wavelength wire to the trunk to act as a reflector element, thus producing a cardioid pattern. Oriented away from the local airport it produces some gain towards the airport and it gives good results.
UPDATE: A new, multi kilowatt FM broadcast station KLVS just set up operation on 107.300 mc. It is about 6 miles away and it completely overloads that little preamp generating intermodulation products. I now hear that FM station all over the VHF aircraft band; what to do?
I put some transmission line theory into practice by installing a quarter wave length open-ended coax stub right at the antenna base (upstream of the preamp.) Correcting for the line 0.66 Velocity Factor. Theory says this will present a “short circuit” at the antenna base-shield connection to (only) 107.300 mc signals before they hit the preamp.
The RG-58C/U coax stub exits the rain cap and extends away from the antenna. Some quick work with an antenna analyzer made easy work of cutting the stub to the correct length. I reinstalled the antenna; the stub completely eliminated the problem caused by that overload condition. Presto, Theory becomes practice. The squirrels still like eating the PVC rain cap.
I have found that 26 gauge gray teflon wire works very well for low-contrast dipoles when the backdrop is the sky. If the background is primarily trees, I have found that brown is better than green insulation. Teflon is shiny and will catch the sun, highlighting an otherwise stealthy antenna. Rubbing the wire between 2 sheets of 400 grit sandpaper before launching roughs up the teflon surface, stopping sun glint.
Remote campsite antennas: Here is an 18 AWG 80 meter dipole rigged at a forest camp. Painted coax, thin, non reflective wire, strung close to the trees. Basic stuff..
“Try to look unimportant – they may be low on Ammo.” Murphy’s Laws of Combat
Keeping bush antennas concealed avoids questions. A good (and laid-back) friend of mine was confronted by an irate Park Ranger for putting up a Field Day wire antenna in “his” trees at a State Park. Threatened with a citation, he was ejected from the park and forfeiting his $30 camping fee. Beware. “Hi – I’m just here camping….Nothing to see here, move along.”
Below is one end of an 80 meter “residential” dipole held up by a tree. I usually don’t use end insulators but I did here. 20 pound test monofilament nylon fish line is a better insulator, rides well in the wind and is practically invisible.
Note the support for one end of the dipole made with WD-1A/TT field telephone wire and insulator:
When held up by this 20 pound test monofilament fish line it is all but invisible relative to the antenna wire.
I use insulated wire whenever possible – it avoids direct contact with wet trees /leaves which helps – although not essential for comms. Also for more permanent wire antennas in the trees insulated wire does not chafe as much as bare stranded wire. Especially with Teflon insulation.
The presence or absence of wire insulation affects both the resonant length and impedance of an antenna. However with HF antennas the effect is minimal and can be ignored relative to other considerations such as height and near-field objects.
Stealthy and therefore lightweight, wire antennas like this are somewhat temporary in nature. Kept slack, they ride well in the wind and accommodate tree swaying but they do occasionally break. I repair mine about once a year where a “permanent” antenna made with 14 Ga wire should hold up longer. Design requirement tradeoffs.
Another stealthy workhorse antenna around here is an Inverted L, the base camouflaged with an inverted flower pot, wire up into the trees. This one is made from paralleled conductors of WD-1A/T infantry telephone wire. Strong, thin, light, hard to see. An initial “breadboard” matching circuit experiment: A 28 microhenry roller inductor, 365 pf air variable and an additional 100 pf ceramic doorknob as needed. The capacitor plate spacing is adequate for my setup.
Initial tests with series/shunt variable mica compression capacitors as I experimented with different wire lengths to get the high-current, radiating segment horizontally and in the clear. “Mr. Smith” (Chart) is my friend!
Below: I settled on a final design for this particular antenna: Series, variable inductor with a shunt variable and fixed capacitor on the antenna-end driving a fairly high impedance Inverted L wire antenna. This system includes the important “ground bounce” radial ground wire buried under the antenna to enhance NVIS. It works very well in this dry soil with minimal transmission line losses.
Here with an adjustable feed point matching circuit to enable occasional frequency/band changes. Designed around a series/shunt variable L/C network protected inside a flower pot. With the coax cable and ground radials buried, pretty innocuous. Nothing to see here, move along.
This antenna is a really good performer for my main interest in 0-400 km “NVIS” regional comms with my buddies using my 10 watt military CW field sets.
Above: The Mark 1, Mod 0 Carbon-based, all terrain, night vision capable, 4X4, autonomous, high-altitude HF Wire antenna installation system. I have the highest HF antennas in the neighborhood.
I have found that almost any piece or wire or metal will work as an effective antenna on HF if it is long enough and reasonably in the clear. This isn’t rocket surgery…You certainly don’t need to buy a commercial wire antenna as long as you have any kind of wire, a matching device and a reasonable ground connection on HF. Unless you only work 20 meters, take that G5RV (or at least the coax section) and bury it for your ground system – you don’t need that compromise-complexity to make lots of contacts.
I have made lots of long range contacts on 80-10 meters from the field with a simple 1/4 wavelength wire thrown up in a tree or laying on top of bushes and another 1/4 wave wire laying on the ground for an ” RF return”. You really don’t need much to communicate when the bands are “open”.
I have used the aluminum siding on my house for an antenna when I was a kid, and I used to load my Hallicrafters HT-40 transmitter into my neighbors “grounded” chain link fence. Not such a great plan, but it worked surprisingly well. If you are trying to break a pileup into VQ9 – Land on a contest weekend, you need something much better, but for casual, reliable ops, simple works just fine!
Below is a photo of my portable, non-stealthy 1/2 wave PVC J-Pole antenna sitting on the ground. But that’s not the point of this photo. Can you see the horizontal wire above and behind it? Didn’t think so…Difficult for the camera – or eye to see.
For some more info on temporary “bush antennas”, see Portable Field Antenna Kit elsewhere on this blog. It describes what will come in handy in making stealthy or otherwise useful antennas for Boonie Ops.
Below is a somewhat stealthy (thin, olive drab, no feedline) EFHW antenna. I’m not a fan of EFHW antennas for my purposes but they can work for random contacts. (Take a look here for my experiences and findings regarding the EFHW: N6CC EFHW Antennas )
This 132′ one includes a 49:1 impedance matching transformer for low power portable work on the low HF bands. I still need to field-harden the transformer assembly with a suitable enclosure.
In a hurry to make your comm window? Try these: Field Expedient Antenna Systems
Speaking of “clandestine comms”, you may need a stealthy, reliable radio to go along with your stealthy antennas…This one is a tried and proven winner.
The RS-6 agent radio
Or for more tactical field ruggedness, this long range Recon set: AN/TRC-77 radio set
Or my personal favorite for covert ops, AN/GRC-109 Special Forces radio set
Another option that takes some equipment and planning is to actually locate the radio and antenna remotely from your operating position: The assistant Radio Operator setting up the AN/PRC-47 in the forest.
Above: An AN/PRC-47 HF SSB/CW transceiver operated by an AN/GRA-6 remote control set over infantry field telephone wire. The radio can be up to 2 miles from the operator and can be remotely turned on or off without a local operator. On this trip the operating position (campsite) with the remote was about 100 meters from here…That system works great if you can swing it. Easy to conceal if necessary. Also keeps artillery away!