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#1
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Antenna informtion needed
I have a bent whip antenna that I'm about to mount in a friends Lancair
and I'm wondering if I should see any connectivity between the center conductor of the BNC connector to the stainless steel element? The ohmmeter shows nothing. Is this thing have capacitance coupling? Thanks Dave |
#2
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If you are talking about one of those old 3/16" stainless steel whips, yes you
do need a connection between the center pin of the RF connector and the element. You also need a ground plane directly at the base of that antenna if you are mounting the antenna in a plastic location. If you plan on using that antenna with a good radio, do yourself a favor and save your friend a lot of grief by using the antenna as a doorstop. The VSWR at the band edges with that sucker is something to behold. Remember, that antenna was designed back in the days when 90 channels (118-127 on 0.1 MHz. spacing) was just about airline quality. Jim dave shared these priceless pearls of wisdom: -I have a bent whip antenna that I'm about to mount in a friends Lancair -and I'm wondering if I should see any connectivity between the center -conductor of the BNC connector to the stainless steel element? - -The ohmmeter shows nothing. -Is this thing have capacitance coupling? - -Thanks - -Dave Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#3
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Hey Jim Thanks!
Maybe I should just build him a generic dipole out of wire or copper tape. How do you get the bandwidth wider, use a wider conductor? Also, my only vswr meter is a old radioshack one for use in the 10 meter range, can I retrofit it to work in the 120 mhz area? Thanks as always Dave Jim Weir wrote: If you are talking about one of those old 3/16" stainless steel whips, yes you do need a connection between the center pin of the RF connector and the element. You also need a ground plane directly at the base of that antenna if you are mounting the antenna in a plastic location. If you plan on using that antenna with a good radio, do yourself a favor and save your friend a lot of grief by using the antenna as a doorstop. The VSWR at the band edges with that sucker is something to behold. Remember, that antenna was designed back in the days when 90 channels (118-127 on 0.1 MHz. spacing) was just about airline quality. Jim dave shared these priceless pearls of wisdom: -I have a bent whip antenna that I'm about to mount in a friends Lancair -and I'm wondering if I should see any connectivity between the center -conductor of the BNC connector to the stainless steel element? - -The ohmmeter shows nothing. -Is this thing have capacitance coupling? - -Thanks - -Dave Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#4
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"dave" wrote in message news:%10ec.6115$_K3.46405@attbi_s53... Hey Jim Thanks! Maybe I should just build him a generic dipole out of wire or copper tape. How do you get the bandwidth wider, use a wider conductor? Also, my only vswr meter is a old radioshack one for use in the 10 meter range, can I retrofit it to work in the 120 mhz area? It depends, some of those CB vswr meters were pure junk, and some are OK for the VHF range. The best way to tell is to check it on a couple of known good installations and also against a dummy load if you have access to one. If the meter reads in watts, ignore that part; the power will read high at higher frequencies. Vaughn |
#5
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On Sat, 10 Apr 2004 16:19:25 -0700, Jim Weir wrote:
The VSWR at the band edges with that sucker is something to behold. Remember, that antenna was designed back in the days when 90 channels (118-127 on 0.1 MHz. spacing) was just about airline quality. Respectful, question, Jim: If he has a transistor output stage. . . . (etc. etc.) Don |
#6
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dave
shared these priceless pearls of wisdom: -Hey Jim Thanks! - -Maybe I should just build him a generic dipole out of wire or copper tape. -How do you get the bandwidth wider, use a wider conductor? That is correct. The "fatter" the conductor the wider the bandwidth. You can do a pretty fair job with two or three strands of bare copper wire (stripped romex) in parallel for each arm of the dipole. The problem is that the fatter the wire (or the apparent wire) the shorter will be the elements. I know that ½" copper tape centers up pretty nice with a length around 20½". If you use bare copper, you might cut one of them a half inch longer and one of them half an inch shorter and see what happens. - -Also, my only vswr meter is a old radioshack one for use in the 10 meter - range, can I retrofit it to work in the 120 mhz area? 9 out of 10 of those old CB VSWR meters aren't worth a darn in the aircraft band. It is catchascatchcan as to whether yours is going to be "good" or not. One easy way to tell is to calibrate it with a few known good antenna installations and see if the CB meter comes even CLOSE to a known good meter. Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#7
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Double whammy with a solidstate output stage. Back in the old vacuum tube days
we just said to hell with the reflective loss, let the RF output tube handle it. So the plate glowed cherry red for a few seconds, no big deal. Solid state "plates" don't glow cherry red but for a microsecond or so and that snap you just heard was $100 worth of frenchfried silicon. So we got clever in our old age and built in "VSWR Protection Circuits", which do nothing more than reduce the available power in the transmitter output to something the transistors can handle. You reduce the transmitted power by reflection and then reduce it even further by the protection circuit. Ain't no winnin' with a narrowband antenna and a 760 channel radio. Jim Don Tuite shared these priceless pearls of wisdom: - - -Respectful, question, Jim: If he has a transistor output stage. . . . -(etc. etc.) - -Don Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#8
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Ok that brings an interesting thought,
When building a multiband antenna, multiple elements can be attached to the same feed point and the element cut closest to the resonant freq will radiate. I did this with an old HF radio for 20, 40 and 80M. Is that what you're doing with the three strands of wire? One say 19.5, the next 20 and the longest at 20.5 inches? One should be resonant somewhere in the band? Also, I read that should I use a torid (SP) and put a loop of coax to keep reflections down. Sound good? If I don't have a vswr meter around should I be pretty safe using this configuration? Thanks for the help! Dave Jim Weir wrote: dave shared these priceless pearls of wisdom: -Hey Jim Thanks! - -Maybe I should just build him a generic dipole out of wire or copper tape. -How do you get the bandwidth wider, use a wider conductor? That is correct. The "fatter" the conductor the wider the bandwidth. You can do a pretty fair job with two or three strands of bare copper wire (stripped romex) in parallel for each arm of the dipole. The problem is that the fatter the wire (or the apparent wire) the shorter will be the elements. I know that ½" copper tape centers up pretty nice with a length around 20½". If you use bare copper, you might cut one of them a half inch longer and one of them half an inch shorter and see what happens. - -Also, my only vswr meter is a old radioshack one for use in the 10 meter - range, can I retrofit it to work in the 120 mhz area? 9 out of 10 of those old CB VSWR meters aren't worth a darn in the aircraft band. It is catchascatchcan as to whether yours is going to be "good" or not. One easy way to tell is to calibrate it with a few known good antenna installations and see if the CB meter comes even CLOSE to a known good meter. Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#9
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dave
shared these priceless pearls of wisdom: -Ok that brings an interesting thought, -When building a multiband antenna, multiple elements can be attached to -the same feed point and the element cut closest to the resonant freq -will radiate. -I did this with an old HF radio for 20, 40 and 80M. Yes, but you will clearly note that you did not run the elements very close together, or you might just as well have used a single piece of wire. The elements were separated, and the more separation the better the multiband antenna worked. However, here you are working with a single frequency, and if the wires are spaced roughly a wire diameter apart, the apparent bandwidth is outrageously good. -Also, I read that should I use a torid (SP) and put a loop of coax to -keep reflections down. Most folks that use toroids as baluns (actually as chokes) use a toroid that just slips over the coax and a couple-three of them close to the antenna. This makes a single-turn coil for each toroid. If you've got the room to use a large toroid and several loops through it, go for it. Embedding them in the structure doesn't allow me to do my designs like that {;-) Don't forget -- the toroid material must be "good" at the frequency of interest, or all you've done is put a steel nut around the coax, and not a choke. Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#10
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HI Dave- This is a quarter wave whip- must be mounted on metal- shield
to ground and center conductor to whip- the whip is not at ground potential- Good luck, KJ dave wrote in message news:n1%dc.5551$_K3.46068@attbi_s53... I have a bent whip antenna that I'm about to mount in a friends Lancair and I'm wondering if I should see any connectivity between the center conductor of the BNC connector to the stainless steel element? The ohmmeter shows nothing. Is this thing have capacitance coupling? Thanks Dave |
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