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Intentional repeater jamming and how to deal with it

repeater jamming repeater jamming

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#41 jec6613

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Posted 25 September 2020 - 03:26 PM

Is the there that much difference in sensitivity between a loop and rhombic?

Usually a minimum of 14 dBd better. A loop antenna is basically a dipole with its ends curved around - for df work though the pattern of a loop lends it to be compact and it has slightly more gain directly broadside than a pure dipole and a more even falloff as you move side to side, so you can locate better.

About the only thing with more directional gain than a rhombic is a curtain array, where an array of dipoles and reflectors an beam a shortwave signal from Alaska to cover all of the Americas, or a caged dipole array - Duga (the Russian woodpecker) is a caged dipole, for instance.

#42 UHFJim

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Posted 25 September 2020 - 07:05 PM

Split tone may be the way to go. Helps stop radios with tone scanning features from picking up the input tone, too.

I have used reverse DCS on portable machines during emergencies, too.

 

Best way to go.  Smart repeater owners slam the jammers by using a PL or DPL for input (do not list the input on myGMRS), and use a different PL/DPL for receive.  



#43 berkinet

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Posted 26 September 2020 - 08:03 AM

Usually a minimum of 14 dBd better. A loop antenna is basically a dipole with its ends curved around - for df work though the pattern of a loop lends it to be compact and it has slightly more gain directly broadside than a pure dipole and a more even falloff as you move side to side, so you can locate better.
...

Thanks for the information. But, just to be sure I understand. Are you saying the rhombic typically has 14dBd more gain than a loop antenna? But, the loop may still be better suited for DFing because of the nulls on each side?


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#44 JohnE

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Posted 26 September 2020 - 09:16 AM

Split tone may be the way to go. Helps stop radios with tone scanning features from picking up the input tone, too.

I have used reverse DCS on portable machines during emergencies, too.

you would be surprised at how many inverse DPL's correlate to a regular DPL and work, if you have the right equipment.

just sayin'


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#45 berkinet

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Posted 26 September 2020 - 09:23 AM

Security through obscurity is no security at all. Just sayin'.


Everything we hear is an opinion, not a fact. Everything we see is a perspective, not the truth.

-- Marcus Aurelius


#46 marcspaz

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Posted 26 September 2020 - 03:47 PM

If I want/need security, I sure as heck won't be on GMRS. LOL. I'm just looking for ways of keeping lazy people from busting chops.

#47 jec6613

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Posted 26 September 2020 - 03:59 PM

Thanks for the information. But, just to be sure I understand. Are you saying the rhombic typically has 14dBd more gain than a loop antenna? But, the loop may still be better suited for DFing because of the nulls on each side?

 

A loop is generally better for DF because it has a very even radiation pattern, slowly rolling off from peak in a predictable way, and is deaf only end-on - gain is slightly better than a simple dipole.  A rhombic has insane gain, but is extremely directional and basically deaf outside of its narrow forward lobe, and the secondary lobes off at 90 degrees to either side.  That of course makes it excellent for fixed installations, and its ability to fold up very small for UHF and upper VHF is another large benefit.

 

Rhombics fell out of favor mostly because of how large they need to be when set up - four sides of 1.5-2 wavelengths is pretty beefy even on the 2M band, and ideally half a wavelength above ground.  Not a problem for UHF, but big trouble once you start getting towards lower VHF, such as 6m, where each leg is 30 feet long at minimum, and about 10 feet off the ground, and for 160m HF you need 700 foot long sides and it to be about 270 feet in the air ... not ideal.  Once the transoceanic cables opened to replace the HF radio links, they've mostly been a forgotten technology - Yagis are slightly less gain, but you can turn them on a rotator, even for HF bands, and height above the ground is much less important.  Some amateur operators with enough space and finances still do use them though, since they can get around the world on the 20, 40, and 80 meter bands even when they're closed like they are now.


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#48 n1das

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Posted 26 September 2020 - 05:11 PM

you would be surprised at how many inverse DPL's correlate to a regular DPL and work, if you have the right equipment.

just sayin'

 

No special or "right equipment" is needed.  If a radio has DPL capability, it already has inverted DPL capability.  With one exception (D172N), every code in the table of 104 standard DCS codes has its inverse somewhere in the table.  For example, codes 125 and 365 are inverses of each other.  D125I is the same as D365N and vice versa.  I've been using 411 (normal) (D411N) and its inverse is 226 (normal).  D411I corresponds to D226N and vice versa.  Normal and Inverted simply refers to the polarity of the DCS waveform. By convention, a logic 1 is represented by a positive carrier shift and a logic 0 is represented by a negative carrier shift.  This is considered Normal polarity for a DCS waveform.

 

A DPL word is a 23-bit cyclic Golay pattern with a 12 bit codeword (23,12).  The 12 bit codeword is formed from the 12 least significant bits of the 23 bit DPL word.  The 12 bit codeword consists of a fixed octal 4 (100 binary) followed by the 3 octal digits that you can program.  Notice that the available DPL codes are octal (base 8) numbers.  The 11 most significant bits are error correcting code bits generated by the Golay algorithm from the 12 codeword bits. The 23 bit DPL word is transmitted repeatedly over the air at 134.4 bits/second.  The least significant bit is transmitted first, resulting in the DPL word being played out backwards over the air.

 

Here is the best technical description of DCS/DPL operation that I've been able to find:

http://www.onfreq.com/syntorx/dcs.html

 

Also check out:

https://wiki.radiore...m/index.php/DCS

 

In my case of dealing with a repeater jammer who didn't have DPL capability and didn't have a clue and thought he knew all there is to know about radio, DPL also stands for "Definitely Prevents Losers" LOL.


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David Sterrett, N1DAS

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Ham [HA] = N1DAS (2/1984)

GMRS [ZA] = KAE9013 (12/1992)

 






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