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Old 12-09-2012, 02:11 AM
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Exclamation Covert Communications



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As anyone who has read the SS-0131 or MCRP 3-40.3C knows; protecting what your transmitting is just as important as who you are transmitting it to; you don't need "Charlie" hearing every dad'blamed transmission because you're using pop's CB with a 2000W "kicker" while you scream into the mic at uncle Joey in the next county over...

Enter NVIS and the Half Rhombic...

NVIS is an acronym for NEAR VERTICAL INCIDENCE SKYWAVE propagation, essentially it is where you "shoot" your signal straight up and it comes straight back down in a sort of umbrella-shape lobe that effectively limits your comm to a certain area thereby excluding spurious transmissions to those, let's say, undesirables who don't need to hear it...

"NVIS, is a radio-wave propagation method that provides usable signals in the range between groundwave and skywave distances (usually 30 to 400 miles, or 50 to 650 km). It is used mainly for military and paramilitary communications and by radio amateurs. The radio waves travel upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction fails to occur and if it is too low absorption reduces the signal strength."

"Significant increases in communication will be realized when both the transmitting station and the receiving station use NVIS configuration for their antennas.

NVIS is most useful in mountainous areas where line-of-sight propagation at VHF or UHF frequencies is ineffective or when the communication distance is beyond ground wave (more than 50 miles, 80 km) and less than sky-wave (300 to 1500 miles, 500 to 2500 km)."


The Half Rhombic is a vertical antenna with a terminating resistor over a counterpoise, giving even distribution of voltages over a wide range of frequencies, here is one example





The Half Rhombic is easy to set up, field expedient, and light weight...

"The vertical half-rhombic antenna is a version of the long wire
antenna that uses a single center support. Easily constructed,
this antenna has a narrow width (as wide as the center support
guys), which allows several to be installed in a relatively
narrow area. The vertical half-rhombic antenna radiates a mediumto
low-angle signal, making it a good choice for medium- to longrange
sky wave circuits. Normally, the 500-foot version is the maximum
length of antenna that most tactical situations will allow;
however, the vertical radiation pattern for a 1,000-foot version is
included, so that if the opportunity exists, the antenna can be used
for excellent results."




"HF NVIS COMMUNICATIONS
NVIS propagation is simply sky wave propagation that uses antennas
with high-angle radiation and low operating frequencies. Just as
the proper selection of antennas can increase the reliability of a
long- range circuit, short-range communications also require proper
antenna selection. NVIS propagation is one more weapon in the
communicator’s arsenal.
To communicate over the horizon to an amphibious ship on the
move, or to a station 100 to 300 kilometers away, the operators
should use NVIS propagation. The ship’s low take-off angle
antenna is designed for medium and long-range communications.
When the ship’s antenna is used, a skip zone is formed. This skip
zone is the area between the maximum ground wave distance and
the shortest sky wave distance where no communications are possible.
Depending on operating frequencies, antennas, and propagation
conditions, this skip zone can start at roughly 20 to 30 kilometers
and extend out to several hundred kilometers, preventing communications
with the desired station.
NVIS propagation uses high take-off angle (60° to 90°) antennas to
radiate the signal almost straight up. The signal is then reflected
from the ionosphere and returns to Earth in a circular pattern all
around the transmitter. Because of the near-vertical radiation angle,
there is no skip zone. Communications are continuous out to several
hundred kilometers from the transmitter. The nearly vertical angle
of radiation also means that lower frequencies must be used. Generally,
NVIS propagation uses frequencies up to 8 MHz.
The steep up and down propagation of the signal gives the operator
the ability to communicate over nearby ridge lines, mountains, and
dense vegetation. A valley location may give the operator terrain
shielding from hostile intercept and also protect the circuit from ground wave and long-range sky wave interference. Antennas used for NVIS propagation need good high take-off angle radiation with very little ground wave radiation."





NEXT WE WILL DISCUSS HOW TO HIDE YOUR ANTENNAS... STAY TUNED...
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Old 12-09-2012, 02:39 AM
PLA PLA is offline
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Very nice, but largely wasted here

You'll find unfortunately most people here dont search. already "know" everything and are looking for someone to reinforce the bad decision they have already made about bubble pack wally world radios and "secure tri square" that are so secret squirrel no body but them knows about it.

Still, a commendable effort
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Old 12-09-2012, 02:54 AM
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Quote:
Originally Posted by PLA View Post
Very nice, but largely wasted here

You'll find unfortunately most people here dont search. already "know" everything and are looking for someone to reinforce the bad decision they have already made about bubble pack wally world radios and "secure tri square" that are so secret squirrel no body but them knows about it.

Still, a commendable effort
hmmm we'll see....

I intend to change this...

There is smug ignorance, a gross oversimplification and caricature that serves as an analytical understanding of communications today, is the common intellectual currency.

Old 12-09-2012, 04:12 AM
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Quote:
Originally Posted by PLA View Post
Very nice, but largely wasted here

You'll find unfortunately most people here dont search. already "know" everything and are looking for someone to reinforce the bad decision they have already made about bubble pack wally world radios and "secure tri square" that are so secret squirrel no body but them knows about it.

Still, a commendable effort
I think I've been insulted?
But I'm too smug and arrogant to admit it !
Old 12-09-2012, 04:17 AM
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Personally I am a big fan of NVIS and have been studying it trying to come up with a method of doing it covertly from my full size truck while moving...so far no go but I am not ready to give up yet.

At the fixed station I am a bit lazy and have little space so I use a G5RV at 20ft for both 40m & 80m with a reflector under it, it works for NVIS but is certainly not optimal.
Old 12-09-2012, 04:35 AM
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Please excuse my lack of knowledge in the matter of HAM radio. But can you use this with a tech, license or does it require something higher? Also is there a particular freq. that this works the best at? I'm sorry I have gone as far as to get my license, I haven't even gotten a radio yet.
Old 12-09-2012, 04:49 AM
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Quote:
Originally Posted by paramilusmc View Post
Please excuse my lack of knowledge in the matter of HAM radio. But can you use this with a tech, license or does it require something higher? Also is there a particular freq. that this works the best at? I'm sorry I have gone as far as to get my license, I haven't even gotten a radio yet.
NVIS normally doesn't work below 40m, typically the rule of thumb is 40m daytime and 80m at night.

I am not up to speed on the tech license and if you can do any 40m or 80m work...thinking you can't but the General ticket is very easy to get, I did both my tech and general at the same time and completed both in less than an hour.
Old 12-09-2012, 04:57 AM
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Quote:
Originally Posted by fullofit View Post
NVIS normally doesn't work below 40m, typically the rule of thumb is 40m daytime and 80m at night.

I am not up to speed on the tech license and if you can do any 40m or 80m work...thinking you can't but the General ticket is very easy to get, I did both my tech and general at the same time and completed both in less than an hour.
That's what I thought. I will attempt to get up to speed so that I can partake. Thanks
Old 12-09-2012, 05:05 AM
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ki4clz
Nice job explaining NVIS, especially the Half Rhombic...that antenna looks interesting to build and test
Old 12-09-2012, 06:15 AM
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"hmmm we'll see....

I intend to change this...

Again I applaud your efforts. In a week there will be 2 threads started about 50 mile handhelds and why be a HAM licencee.

And every week after that.....

After reposting a few dozen times you will understand what I am talking about.

Nice post, again
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Old 12-09-2012, 12:13 PM
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Quote:
Originally Posted by paramilusmc View Post
Please excuse my lack of knowledge in the matter of HAM radio. But can you use this with a tech, license or does it require something higher? Also is there a particular freq. that this works the best at? I'm sorry I have gone as far as to get my license, I haven't even gotten a radio yet.
Unless you know CW, your locked out of those bands as a Tech. Just get the general and you will be set.
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Old 12-10-2012, 03:17 AM
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Mity Mite radios, cost about $29, very easy high school level builds. My son built 1 using the utube instructional video. I bought the extra crystal set so I had the full range of CW in all the bands available. Pre cut your antennas for each freq, and no tuner needed. Buy a key and earphone(crystal so it needs no power) and you are sending Entire set fits in shirt pocket, recurring expenses are 9 volt batteries.
Old 12-10-2012, 05:13 AM
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NVIS makes a lot of sense for survival comms as it can provide roughly statewide coverage; with non-NVIS antennas the skip zone could be a significant problem.

NVIS is not very covert, since civilian users are not allowed encryption.

Half rhombic does not seem to be a very good antenna for NVIS. Resistor terminated antennas can be broadband with low SWR, but so is a dummy load and the resemblance is not coincidental - that is a dummy load sitting at the other end of the antenna. Around 32% efficient on 80 and 40 meters. And a radiation pattern leaves something to be desired. -12dB gain (loss) vertically. That means with 100W transmit power you are only getting 6W skyward. Too much gain at low angles on NVIS frequencies compared to NVIS angles means you will pick up interference from lightning and DX stations. The terminated antenna can work all HF bands, though, without a tuner and without the transceiver folding back due to SWR but it might not actually get as much signal out as an antenna with poor SWR.
www.korpi.biz/hr.pdf
If instead of putting a resistor terminator at the end of the line, you put a high power 10dB attenuator in series with the transmitter, you would see a maximum SWR of 1.22:1 no matter how bad the antenna and a 6dB antenna would make the SWR 1.7:1 or better. And probably get as much signal launched into the air. A dummy load built as a 3dB attenuator (with a switchable 50ohm load at the output end) could probably double as a reasonable, if inefficient, emergency antenna "tuner"; loss would be significant compared to a good antenna but not compared to your terminated "would you believe it is all band" antennas.

EDIT: the rhombic antenna efficiency does improve on higher frequency bands. It is interesting to note that an 80m vertical delta loop (see ARRL link) would be about the same size as the rhombic, but taller, and have much better performance on all 4 NVIS bands but not on higher frequency bands. Those interested in multiband without a tuner might want to look at the off center fed dipole, the VA2ERY Clothesline antenna in Jul 1998 QST (adjustable off center fed dipole), or the Yo-Yo clothesline reel antenna (variable length dipole).
http://p1k.arrl.org/pubs_archive/94569
http://forums.qrz.com/showthread.php...nslulated-Wire

The sloping V is another terminated antenna with low efficiency.

Currently, I considering an 80m full wave loop skywire antenna about 15ft off the ground. Resonant on odd an even harmonics of design frequency. Gain in NVIS direction on NVIS bands but pattern is more DX oriented on the non-NVIS bands.
Here are some tests on one but it is a 40m loop, not 80.
http://www.qsl.net/kp4md/kp4mdnec2.htm
80m loop:
http://dk5ec.de/deltaloop-eng.htm
12dB more vertical gain at 80MHz and about 2db more at 40MHz compared to rhombic and less noise near the horizon. And while it is frequently not shown, radiation pattern and gain is about the same at 60m as 80m and vertical gain at 160m is about the same as 40m (with less noise from the horizon).
http://www.arrl-sc.org/Tech%20Presen...e%20(NVIS).ppt

High Sunspot activity:
- Daytime: 60m, 40m, and 30m
- Nighttime: 60m and 80m
Low Sunspot activity:
- Daytime: 80m,60m, or 40m
- Nightime: 80 or 160m
http://www.arrl-sc.org/Tech%20Presen...e%20(NVIS).ppt

40m was recently closed to NVIS for more a year and a half (some say 3 years) due to low sunspot activity above 30degrees north latitude.
http://www.w0ipl.net/ECom/NVIS/nvis.htm


Skip distance and maximum reliable range table by frequency:
http://www.w8ne.com/Files/RadioRangeTableII.xls
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Old 12-10-2012, 07:29 AM
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What's the "covert" angle of NVIS? I don't see any indication that the signal is hidden from Charlie within the umbrella. What it arguably can do is make it more difficult to triangulate your position, but not impossible.
Old 12-10-2012, 08:25 AM
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Quote:
Originally Posted by fullofit View Post
Personally I am a big fan of NVIS and have been studying it trying to come up with a method of doing it covertly from my full size truck while moving...so far no go but I am not ready to give up yet.

At the fixed station I am a bit lazy and have little space so I use a G5RV at 20ft for both 40m & 80m with a reflector under it, it works for NVIS but is certainly not optimal.
The most inexpensive technique is using a notch antenna. Almost everything else involves stopping the vehicle.

http://www.tactical-link.com/nvis3.htm

You can use a loop antenna like this one, but buying it is usually damn expensive.

http://www.smc-comms.com/nvis_transp...op_antenna.php

Quote:
I don't see any indication that the signal is hidden from Charlie within the umbrella
OMG!! flashback!!!!!
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Old 12-10-2012, 09:01 AM
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Quote:
Originally Posted by Sky1950 View Post
The most inexpensive technique is using a notch antenna. Almost everything else involves stopping the vehicle.

http://www.tactical-link.com/nvis3.htm

You can use a loop antenna like this one, but buying it is usually damn expensive.

http://www.smc-comms.com/nvis_transp...op_antenna.php



OMG!! flashback!!!!!
From a dfferent page on the Tactica Link web site.
http://www.tactical-link.com/field_deployed_nvis.htm

Check out the summer of 1990 test.

A 15 foot whip deployed horizontaly getting the same result as a 100 foot end fed. I don't see why it couldn't be used on the move.
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Old 12-10-2012, 09:07 AM
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Originally Posted by Tweightwee View Post
From a dfferent page on the Tactica Link web site.
http://www.tactical-link.com/field_deployed_nvis.htm

Check out the summer of 1990 test.

A 15 foot whip deployed horizontaly getting the same result as a 100 foot end fed. I don't see why it couldn't be used on the move.
Yeah, our commo goys used to tie the whip antenna back toward the rear of the jeep, trying to redirect the orientation of the RF lobe, But I didnt mention it because trailing a whip like that just seemed a little unstable to me. Plus I never saw any of my guys doing it mobile back then, it was always when the jeep was stationary. worth a try I guess.
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Old 12-10-2012, 10:20 AM
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Long wire antenna are a great method of getting a signal out .
A sling shot fishing line and wire will gve you a great advantage, the more you learn about radio.
Secondly learning how to make beem antenna to focus signal in a particular direction and null it almost completely behind can also isolate your conversation ,as well as using pl tones and off setting the send recieve frequencies.
Even with out a ham license , one could send on a CB frequency the companion would be recieving on and the companion could be transmitting on family radio you are recieving on . Cross band communication.
Any trick you use can work for a while ,but be assured ,if you are stepping on some ones toes causing trouble , Amatures will be the first to find you if you are operating in their teritory of frequency range .
Some of the reason the sheriffs radios are trunked is to automaticlly change frequency to keep the general public out of their business.
Problem is scanners already monitor them any way.
It is a good idea to keep in mind that what you say can and will be used against you .
Be it CB family radio or amature radio or business bands ,even cell phone traffic is easily tapped , not many think about that either.
You can use other languages , but for amature radio, one must identify in english, world wide .
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Old 12-10-2012, 10:43 AM
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"It is a good idea to keep in mind that what you say can and will be used against you .
Be it CB family radio or amature radio or business bands ,even cell phone traffic is easily tapped , not many think about that either"

All of you who are reading this , there were never truer words spoken than that few above. You are being monitored, and anything you say can and will be used against you...
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Old 12-10-2012, 06:08 PM
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Quote:
Originally Posted by paramilusmc View Post
Please excuse my lack of knowledge in the matter of HAM radio. But can you use this with a tech, license or does it require something higher? Also is there a particular freq. that this works the best at? I'm sorry I have gone as far as to get my license, I haven't even gotten a radio yet.
Sho' Nuff...!

The Tech License has privileges on the low bands too...!
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