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Coax length a function of antennae length or just run it??
- Thread starter McFly
- Start date
Short runs...Yes, but in the case of transponder cables, you can get them too short. If the cable is less than about 18" it can cause the transponder to reply to itself. Normally the transponder cable should at least 2'. Also the higher freguency cables (transponder, GPS, etc.) should not be bundled with com antenna cables because of possible cross-talk interference.
mlw450802
Well Known Member
BALUN or not?
Sort of in the same thread...
I know that if you connect coax to a symmetric dipole, a balun helps to match impedance to the asymmetric coax.
Is a balun necessary or desirable when connecting coax to a whip on ground plane? Both are sort of asymmetric (coax and whip) but someone out there must be really knowledgeable in antennaes and can provide a definitive answer.
Thanks
-Mike
Sort of in the same thread...
I know that if you connect coax to a symmetric dipole, a balun helps to match impedance to the asymmetric coax.
Is a balun necessary or desirable when connecting coax to a whip on ground plane? Both are sort of asymmetric (coax and whip) but someone out there must be really knowledgeable in antennaes and can provide a definitive answer.
Thanks
-Mike
RV7Guy
Well Known Member
Coax lengths
First, I have no specific knowledge of the "black magic" aspect of this stuff. I do have knowledge of what I was told by Garmin regarding lengths and connectors etc..
Mel is right on and this is exactly what Garmin told me regarding the transponder.
The guy I talked to said to make sure the connections are solid, get a proper spread on antennas and don't worry about the rest. Also make sure all grounds are properly done.
I personally put bnc connectors at the tips and root rib for wing tip and wing disconnects. I was originally concerned about having two connectors in line. Again the Garmin guy said on a plane the size of an RV it is really difficult to mess things up with coax lengths, connectors etc...
My radios work perfectly. I have Archer antennas for VOR, MB and Comm2. I have a belly mounted antenna for Comm 1 and Transponder. All are clear as confirmed by a back channel check with Phx Tracon and numerous Centers around the SW.
Bottomline, JUST RUN IT!!!! It will work fine.
Mel said:
First, I have no specific knowledge of the "black magic" aspect of this stuff. I do have knowledge of what I was told by Garmin regarding lengths and connectors etc..
Mel is right on and this is exactly what Garmin told me regarding the transponder.
The guy I talked to said to make sure the connections are solid, get a proper spread on antennas and don't worry about the rest. Also make sure all grounds are properly done.
I personally put bnc connectors at the tips and root rib for wing tip and wing disconnects. I was originally concerned about having two connectors in line. Again the Garmin guy said on a plane the size of an RV it is really difficult to mess things up with coax lengths, connectors etc...
My radios work perfectly. I have Archer antennas for VOR, MB and Comm2. I have a belly mounted antenna for Comm 1 and Transponder. All are clear as confirmed by a back channel check with Phx Tracon and numerous Centers around the SW.
Bottomline, JUST RUN IT!!!! It will work fine.
gmcjetpilot
Well Known Member
Losses
There is loss for every foot or run and every connector, so minimize these if you can. I don't know the percentage but its not totally trivial. Also the RG400 is the better stuff as oppose to the typ RG58 coax.
There is loss for every foot or run and every connector, so minimize these if you can. I don't know the percentage but its not totally trivial. Also the RG400 is the better stuff as oppose to the typ RG58 coax.
For these frequencies, output powers, and signal types, shorter runs of coax ARE generally better. MOST importantly is using high quality cable. I don't care if RG-58 is 50ohm cable or not. This cable IS NOT suitable for frequencies higher then the CB in your neighbors pickup truck.
Connectors are another issue. HIGH-QUALITY connectors are ESSENTIAL, and PROPERLY connected connectors work a WHOLE BUNCH MORE BETTER then improperly fitted connectors.
Bottom line, use a high-quality (more-expensive) cable that meets the impeadance requirements of the feedline, yet still is manageable in diameter for aircraft useage, and cut it to length, with just a litte extra on both ends in case corrosion or fatique takes the connector out later in life.
Connectors are another issue. HIGH-QUALITY connectors are ESSENTIAL, and PROPERLY connected connectors work a WHOLE BUNCH MORE BETTER then improperly fitted connectors.
Bottom line, use a high-quality (more-expensive) cable that meets the impeadance requirements of the feedline, yet still is manageable in diameter for aircraft useage, and cut it to length, with just a litte extra on both ends in case corrosion or fatique takes the connector out later in life.
Last edited by a moderator:
az_gila
Well Known Member
Check the specs. and the installation instructions
RG-58 is not bad stuff ... you can argue the PVC composition vs. teflon(?) for RG-400, but the performance differences are quite small.
Co-ax specifications are here...
http://www.nemal.com/Catalog/30.pdf#search="rg-58 specifications"
Garmin, in their 430 installation manual calls for GPS antenna cable that has less than 10 db at 1.5 GHz.
The above spec. sheet has RG-58 at around 40 db for 100 ft., but at double the frequency of 3 GHz.
Even at this higher frequency, 10 ft. of RG-58 and a couple of good connectors will meet this requirement, which is good enough for TSO'd installations.
The GNC 430 installation manual has NO restrictions on cable length for COMM, VOR, GS and GPS cables, though they do suggest that the GPS cable be kept to less than 40 ft. - their actual requirement is the 10 db loss.
At the lower comm/vor frequencies, RG-400 is around 4 db loss per 100ft. at 100MHz, while RG-58 is around 5 db loss per 100 ft. - not really much difference for the above cabling.
Transponders - much more interesting...
The Garmin installation manual for the GTX 320 has some specific requirements, which dictate a better co-ax cable.
The antenna must be 3 ft away from the unit - note this is not a cable length restriction, but a straight line distance requirement.
Garmin does have a good electrical theory for this, it is "to prevent RF interference" - direct transmission through the air of 1 GHz high power pulses apparently can exceed their shielding of the GTX-320 box[/I][/B]
There is a 1.5 db at 1.09 GHz (much tighter than above) total loss (includes connectors) requirement for the cable.
RG-400 isn't that great, and if you use it, you have a maximum length of 8.8 ft.
For longer runs, you need better co-ax - RG-304 is 12.5 ft max. - RG-393 is 17 ft. maximum. - these numbers directly from the installation manual.
This is actually important for us, since the ELT and Transponder installations are required to be TSO'd (or to an equivalent specification) per the FARs
From this EAA web site...
http://members.eaa.org/home/homebuilders/faq/1Equipping a Homebuilt for IFR operations.html?
The requirements for transponder equipment and operation are found in 91.215, which has this to say:
(a) All airspace: U.S.-registered civil aircraft. For operations not conducted under part 121 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate, or the appropriate class of TSO-C112 (Mode S).
Note that, while it is required that the transponder equipment meet the performance and environmental requirements of the applicable TSO, it is not required that the equipment be manufactured under a TSO authorization. In theory, this means that you could actually build your own transponder, so long as you can document that it meets the requirements of the applicable TSO. However, the easiest way to be assured that your transponder meets the requirements of 91.215(a) is to install one that has been built under a TSO authorization.
So, the best way to meet the TSO requirement is to buy TSOd equipment (all transponders are) and to follow the manufacturer's installation instructions.
Check the installation instructions for the specific equipment you have. Check the specifications of the material you are using. Garmin is quoted above, your equipment may vary...
Good connectors and good solder joints at these connectors are probably more important.
gil in Tucson
RG-58 is not bad stuff ... you can argue the PVC composition vs. teflon(?) for RG-400, but the performance differences are quite small.
Co-ax specifications are here...
http://www.nemal.com/Catalog/30.pdf#search="rg-58 specifications"
Garmin, in their 430 installation manual calls for GPS antenna cable that has less than 10 db at 1.5 GHz.
The above spec. sheet has RG-58 at around 40 db for 100 ft., but at double the frequency of 3 GHz.
Even at this higher frequency, 10 ft. of RG-58 and a couple of good connectors will meet this requirement, which is good enough for TSO'd installations.
The GNC 430 installation manual has NO restrictions on cable length for COMM, VOR, GS and GPS cables, though they do suggest that the GPS cable be kept to less than 40 ft. - their actual requirement is the 10 db loss.
At the lower comm/vor frequencies, RG-400 is around 4 db loss per 100ft. at 100MHz, while RG-58 is around 5 db loss per 100 ft. - not really much difference for the above cabling.
Transponders - much more interesting...
The Garmin installation manual for the GTX 320 has some specific requirements, which dictate a better co-ax cable.
The antenna must be 3 ft away from the unit - note this is not a cable length restriction, but a straight line distance requirement.
Garmin does have a good electrical theory for this, it is "to prevent RF interference" - direct transmission through the air of 1 GHz high power pulses apparently can exceed their shielding of the GTX-320 box[/I][/B]
There is a 1.5 db at 1.09 GHz (much tighter than above) total loss (includes connectors) requirement for the cable.
RG-400 isn't that great, and if you use it, you have a maximum length of 8.8 ft.
For longer runs, you need better co-ax - RG-304 is 12.5 ft max. - RG-393 is 17 ft. maximum. - these numbers directly from the installation manual.
This is actually important for us, since the ELT and Transponder installations are required to be TSO'd (or to an equivalent specification) per the FARs
From this EAA web site...
http://members.eaa.org/home/homebuilders/faq/1Equipping a Homebuilt for IFR operations.html?
The requirements for transponder equipment and operation are found in 91.215, which has this to say:
(a) All airspace: U.S.-registered civil aircraft. For operations not conducted under part 121 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate, or the appropriate class of TSO-C112 (Mode S).
Note that, while it is required that the transponder equipment meet the performance and environmental requirements of the applicable TSO, it is not required that the equipment be manufactured under a TSO authorization. In theory, this means that you could actually build your own transponder, so long as you can document that it meets the requirements of the applicable TSO. However, the easiest way to be assured that your transponder meets the requirements of 91.215(a) is to install one that has been built under a TSO authorization.
So, the best way to meet the TSO requirement is to buy TSOd equipment (all transponders are) and to follow the manufacturer's installation instructions.
Check the installation instructions for the specific equipment you have. Check the specifications of the material you are using. Garmin is quoted above, your equipment may vary...
Good connectors and good solder joints at these connectors are probably more important.
gil in Tucson
Last edited by a moderator:
az_gila
Well Known Member
No balun needed...
Mike... a quarter wave whip antenna on a perfect ground plane has an impedance of 50 ohms, so it will match the co-ax well. No balun is needed.
This is true for either RG-400 or the non-appreciated RG-58...
This site probably tells you much more than you need.... ... but figure 2 shows this....
http://www.electronics-tutorials.com/antennas/antenna-basics.htm
gil in Tucson
mlw450802 said:
Mike... a quarter wave whip antenna on a perfect ground plane has an impedance of 50 ohms, so it will match the co-ax well. No balun is needed.
This is true for either RG-400 or the non-appreciated RG-58...
This site probably tells you much more than you need....
... but figure 2 shows this....http://www.electronics-tutorials.com/antennas/antenna-basics.htm
gil in Tucson
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Co-ax specifications are here...
http://www.nemal.com/Catalog/30.pdf#search="rg-58 specifications"
http://www.radiobooks.com/products/techinfo/coaxloss.htm
http://www.nemal.com/Catalog/30.pdf#search="rg-58 specifications"
http://www.radiobooks.com/products/techinfo/coaxloss.htm
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robertahegy
Moderator/Tech Counselor
I removed some inflamatory and unnecessary commentary from this string. The information offered was left intact. Offer what info you may have and please refrain from attacking or provoking others who are doing the same.
Roberta
Roberta
