N6CC – by Tim“Never do anything to attract enemy fire – It annoys those around you”
Murphys Laws of Combat.
UPDATED 2/1/13 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! Like many of us, I live in an “antenna challenged” neighborhood but I was fortunately able to get mine “grandfathered” in when I first moved here since the original CC&R/HOA documents did not outright prohibit them at the time. However I keep them very low profile to keep the neighbors happy anyway.
Cover, concealment, camouflage – or hidden in plain sight.
See that 17 foot 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)
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 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, 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 RTV to stop corrosion on that end. This antenna has been in continuous use for the past 15 years. Invisible.
The wood rafters provide adequate “RF insulation”, even when everything is wet – I see minimal change in the SWR / complex impedance and any power loss appears to be negligible, certainly in terms of distant field strength “S Units”. The down spouts are also part of the system and might be a potential personnel contact problem at high power levels but at low power they are not a hazard. If there is concern, inserting a short PVC section between the gutter and downspout will cure that concern. Think it through.
Rain gutter covert HF antenna
Gutter antenna connection “detail”. See the connection? Simple, effective.
Above: This gutter antenna is connected via an upper roof-level downspout. This all-aluminum gutter is fed with coax with an unterminated shield (at this end) to reduce nearby computer hash from inside the house. 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 about 2000 miles with a 100 watt transmitter. It works great and seems to be not affected by rain/wet conditions. Pretty invisible.
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. 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 low power levels on the 80-60-40-30 meter bands they are also RF safe to the building inhabitants (me). 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 feed line is concealed under the 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.
Tree Supports/Camouflage:
Below is another type of covert antenna. This time, it’s a 1/4 wave ground plane for 145 MC. 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 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. Fed by coax 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.
The PVC pipe is barely visible, even up close, the antenna brazing rod elements are also sprayed with flat O.D. paint and they are now invisible.
Wire antennas: 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. Below is one end of an 80 meter dipole held up in a tree. I usually don’t use end insulators – 20 pound test monofilament nylon fish line is a better insulator, rides well in the wind and is practically invisible. I use insulated wire whenever possible – it avoids direct contact with wet trees /leaves which helps – although not essential for comms. “See” below. The antenna is very hard to see unless you are looking for it – easily overlooked. (Occasionally a hummingbird will land on one – entirely blowing my cover….)
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.
Mk1 Mod 0 Antenna Installation System
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 science…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 and bury it for your ground system – you don’t need it 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 ” RFreturn”. 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. Worked pretty 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 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.
http://www.n6cc.com/tactical-antenna-systems