Post by Admin-N8SDR on Jul 24, 2021 16:18:42 GMT -5
N8SDR’s 80 Meter Coaxial Inverted L –
items needed
Approx. 70 Feet of Mini 8 (8X) MUST HAVE .78 Velocity Factor
1 4X4X4 nema outdoor Box
2 chassis mount (long) SO239 connectors
1 pl259 and 8x reducer
1 10-24 X 1 1/2 Stainless Bolt 3 Nuts 4 flat washers
Shrink wrap to fit the Mini 8 coax
Misc. ring terminals to fit above bolt and 1 foot of 12-14 ga. wire- some U/V zip ties and electrical tape and possibly Coax sealer wire for ground radials
Using Mini 8 (8X) with a velocity factor of .78 the bottom section of the 80-meter inverted coaxial "L" should be a piece of RG-8X that is 51' and10-5/16" long with only the top of the coax shorted.
The rest of the antenna should be a piece of RG-8X that is 10' and 3-5/8" long with both of its ends shorted.(the ¼-wave shorted stub)
That gives you a total antenna length very close to 62'-2".
As you well know, the tolerance in the velocity factor, as well as the tolerance in the distributed capacitance, of the actual piece of coax that you use, could cause those dimensions above to vary by several inches.
Building the coaxial Inverted from a 70-foot run of Mini8 (8X) measure 52 Feet from one end. Mark this location (I used a Sharpie and made a colored band around the coax- from that mark measure out 10 Feet 8 Inches and again mark the coax.
At the starting section to the 52-foot mark install a PL259 connector (make sure you remember the reducer)
At the 52 Foot Mark remove aproximeantly 1 to 1 ½ inches of the outer jacket- using a pick gently pull the braid to one side so you can expose the inner center conductor, While bending the coax back on itself remove the jacket from the inner conductor- Tin the inner conductor with solder- and twist the coax to cause the shield to wrap around the inner conductor (making a short if you will) and solder causing the center and shield to be shorted (connected at this point) you want to make this connection strong as it will need to support the end and sections before it as part of the antenna- using Shrink tube to seal this section from the weather- then wrap several inches before and after with electrical tape. (suggestion I cut 2 end sections from a tie wrap and laid them along side the heat shrink before tightly wrapping electrical tape around this portion my though was it may help stop with pulling at this section)this is a weak spot of this antenna and care needs to be taken to avoid pulling orstretching this spot.
At the end of the 10 foot 8 inch section (measure from were you should have the coaxed shorted in the above step again remove approximant 1 inch or so and short the braid/shield to the center conductor and solder – again shrink wrap and seal it with electrical tape (rescue tape would be a better choice for this the above section)
Now you need to find a way to all suspend this end of the antenna ( currently I am using about 10 inches of rope a couple hitch knots 3 UV zip ties and covered that 10 inch section of coax, zip ties and rope with electrical tape around a foot or so I tied an insulator(dog bone) to the rope.
Hang the antenna in an inverted L configuration- keep the vertical section at least 25- 35 feet the rest should be horizontal.
The bottom section of the vertical I have approximately 2 feet off ground.
Using the Nema box drill 2 holes to accommodate the chassis mount SO239 adapters (lay them out so one of them will face straight up this connector will be used to attach out vertical section of antenna- place the other either one a side or bottom depending on how you intend to get your feedline to the antenna ( I chose bottom thus I have a connector centered on the box top and bottom) – then drill a hole on one of the sides that doesn’t not have a connector in order to pass the bolt thru (threads will be facing the outside.
Time to make the connections inside the box- determine which of the 2 SO239 will be used for the vertical antenna connection- you will need to solder a wire from the center pin of that connector to the Outer shell of the other SO239- and then from the feedline section connector center pin you will connect the outside (or shell) connection on the Vertical so239 connector (YES the connections Must be crossed do not connect Pin to Pin and Ground/shell to shell).
Using a small piece of wire attach a ring terminal at the other end will connect to the CENTER PIN of the SO239 used for the vertical section- slip the bolt thru the ring terminal tighten with a nut place a flat washer on top of the nut and pass this bolt through the hole so the threads face the outside this will be used for your grounding – place a flat washer and secure again with a nut and tighten down now place the other two flat washers and nut on the end ( place your ground radials and ground connection wires between the two flat washers .
I have a small backyard but 80 meters is a fun band for local rag chew FT8 or other digital modes and even DX- currently I have a somewhat poor ground radial system- about 6 radials buried just a couple inches below the ground at approximately 15-30 feet in length (the more you have down even if short the better the antenna will perform)
Having had up a standard wire inverted L for many years I am in the process of comparing this Coaxial inverted L – one thing I can tell you – it appears to be very broad banded using the lengths above at 3.5 MHz I have a 1:0:1 swr at the top of the band 4.0 MHz I have a 2.0 SWR, I can probably shorten the (10-8 inches section down to the suggested 10 foot roughly 4 inches and drop the upper end of the bend down slightly.
The Mini 8 made coax antenna should easily handle legal limit so far, no issues at little over 1KW here.
What I have noticed besides the broad bandwidth, seems to be possibly slightly less in receiver noise- Rain does not affect the tunning – and seems to play better so far then my standard Wire Inverted L, I’ll need more time to really test this but as an example
Don N8KYX on average I would receive him at around 10-15 over s9- he is now 20-25 over- and when I was running 700-800 watts on the old antenna I would be about 5 over to him Now at 80 watts I’m now 5 over slightly more and 20+ at the 800 watts or so. For local performance that is a HUGE improvement
A group I tend to jump in with near Ashville Tennessee noticed a stronger signal as well
Then at 6:50 PM Saturday 10/3 right after N8KYX and I got the antenna up I worked 3 Spain station on FT8 back to back with 45 watts normally I would not hear EU stations before sunset (I would hear them after sunset but not before) this could have been a fluke and just good timing and band conditions , but signals reported on both ends were the same -12/-13.
As far as I can tell, this should not be used as a multiband inverted L – I do not think the high impedance and voltage would take long to breakdown the antenna near its current maximum point. I could be wrong but am not willing to find out. I’ll keep those interested posted on more findings so far I don’t see a reason to switch back to the standard Wire made inverted L unless your needing multiband usage- that seems to be along with its probably a much less prone to weather breaking it in wind etc., the only reasons to use one. If you can make the supports good get the vertical section minimum of 25 feet and the horizontal section 25 feet or more and do not need multiband usage the coaxial L is most likely a better performer!
A drawing of the crossover box can be found here
drive.google.com/file/d/10LNcwPKW6Z68SOrP5eWrB6yJYK3hYWwW/view?usp=sharing
Outline drawing of antenna can be found here
drive.google.com/file/d/1lSe7jE4tu4SbMagKr2qz3J0SIn8409kN/view?usp=sharing
I scaled this off the post below-
Subject: Re: 160 meter "inverted L"
View: Complete Thread (2 articles)
Original Format
Newsgroups: rec.radio.amateur.antenna
Date: 1996/07/23
Adam, in the early 80's Coleman Rowland, W4TWW published an excellent
article in CQ on the Coaxial Inverted L For 160 meters. I have built the
antenna and found it to be excellent. In theory the antenna is 1/4 wave
long and is made of a 50 ohm coax section and a 300 ohm twin lead section.
The coax section is an electrical quarter wave (246/FMHz times the
velocity factor) and the 300 ohm twin lead makes up the rest of the
quarter wave.
In reality through trial and error Coleman, found that the total length of
the antenna should be 230/FMHz; and the coax section should be 162.5/FMHz.
For 1.850 MHz this works out to 88 feet for the coax and 37 feet for the
twin lead for a total length of 125 feet.
Short the center conductor to the shield of the coax, and connect this to
the twin lead. The twin lead is shorted at this connection point and also
shorted at the opposite end.
Feed it at the other end of the coax, using 50 ohm coax from your rig and
connect the center conductor of the coax from your rig to the shield of
the coax making the antenna; and the shield of the coax from the rig to
the center conductor of the coax making the antenna. The center conductor
of the antenna coax goes to ground with as many radials as possible.
Mount it as vertical as possible and pull the rest out as horizontal as
possible. If you can go up 88 feet it's good to get the entire coaxial
section vertical, but if you can't it will still work well.
The antenna is very broad and will be useable across the entire 160 meter
band without a tuner. However, it is still fundamentally a vertical so
you need as many radials as you can possibly get.
I later made a change to W4TWW's design which I would recommend. I simply
replaced the 300 ohm twin lead with another piece of 50 ohm coax. It's
much stronger and I saw no difference electrically.
Good luck, Merv
items needed
Approx. 70 Feet of Mini 8 (8X) MUST HAVE .78 Velocity Factor
1 4X4X4 nema outdoor Box
2 chassis mount (long) SO239 connectors
1 pl259 and 8x reducer
1 10-24 X 1 1/2 Stainless Bolt 3 Nuts 4 flat washers
Shrink wrap to fit the Mini 8 coax
Misc. ring terminals to fit above bolt and 1 foot of 12-14 ga. wire- some U/V zip ties and electrical tape and possibly Coax sealer wire for ground radials
Using Mini 8 (8X) with a velocity factor of .78 the bottom section of the 80-meter inverted coaxial "L" should be a piece of RG-8X that is 51' and10-5/16" long with only the top of the coax shorted.
The rest of the antenna should be a piece of RG-8X that is 10' and 3-5/8" long with both of its ends shorted.(the ¼-wave shorted stub)
That gives you a total antenna length very close to 62'-2".
As you well know, the tolerance in the velocity factor, as well as the tolerance in the distributed capacitance, of the actual piece of coax that you use, could cause those dimensions above to vary by several inches.
Building the coaxial Inverted from a 70-foot run of Mini8 (8X) measure 52 Feet from one end. Mark this location (I used a Sharpie and made a colored band around the coax- from that mark measure out 10 Feet 8 Inches and again mark the coax.
At the starting section to the 52-foot mark install a PL259 connector (make sure you remember the reducer)
At the 52 Foot Mark remove aproximeantly 1 to 1 ½ inches of the outer jacket- using a pick gently pull the braid to one side so you can expose the inner center conductor, While bending the coax back on itself remove the jacket from the inner conductor- Tin the inner conductor with solder- and twist the coax to cause the shield to wrap around the inner conductor (making a short if you will) and solder causing the center and shield to be shorted (connected at this point) you want to make this connection strong as it will need to support the end and sections before it as part of the antenna- using Shrink tube to seal this section from the weather- then wrap several inches before and after with electrical tape. (suggestion I cut 2 end sections from a tie wrap and laid them along side the heat shrink before tightly wrapping electrical tape around this portion my though was it may help stop with pulling at this section)this is a weak spot of this antenna and care needs to be taken to avoid pulling orstretching this spot.
At the end of the 10 foot 8 inch section (measure from were you should have the coaxed shorted in the above step again remove approximant 1 inch or so and short the braid/shield to the center conductor and solder – again shrink wrap and seal it with electrical tape (rescue tape would be a better choice for this the above section)
Now you need to find a way to all suspend this end of the antenna ( currently I am using about 10 inches of rope a couple hitch knots 3 UV zip ties and covered that 10 inch section of coax, zip ties and rope with electrical tape around a foot or so I tied an insulator(dog bone) to the rope.
Hang the antenna in an inverted L configuration- keep the vertical section at least 25- 35 feet the rest should be horizontal.
The bottom section of the vertical I have approximately 2 feet off ground.
Using the Nema box drill 2 holes to accommodate the chassis mount SO239 adapters (lay them out so one of them will face straight up this connector will be used to attach out vertical section of antenna- place the other either one a side or bottom depending on how you intend to get your feedline to the antenna ( I chose bottom thus I have a connector centered on the box top and bottom) – then drill a hole on one of the sides that doesn’t not have a connector in order to pass the bolt thru (threads will be facing the outside.
Time to make the connections inside the box- determine which of the 2 SO239 will be used for the vertical antenna connection- you will need to solder a wire from the center pin of that connector to the Outer shell of the other SO239- and then from the feedline section connector center pin you will connect the outside (or shell) connection on the Vertical so239 connector (YES the connections Must be crossed do not connect Pin to Pin and Ground/shell to shell).
Using a small piece of wire attach a ring terminal at the other end will connect to the CENTER PIN of the SO239 used for the vertical section- slip the bolt thru the ring terminal tighten with a nut place a flat washer on top of the nut and pass this bolt through the hole so the threads face the outside this will be used for your grounding – place a flat washer and secure again with a nut and tighten down now place the other two flat washers and nut on the end ( place your ground radials and ground connection wires between the two flat washers .
I have a small backyard but 80 meters is a fun band for local rag chew FT8 or other digital modes and even DX- currently I have a somewhat poor ground radial system- about 6 radials buried just a couple inches below the ground at approximately 15-30 feet in length (the more you have down even if short the better the antenna will perform)
Having had up a standard wire inverted L for many years I am in the process of comparing this Coaxial inverted L – one thing I can tell you – it appears to be very broad banded using the lengths above at 3.5 MHz I have a 1:0:1 swr at the top of the band 4.0 MHz I have a 2.0 SWR, I can probably shorten the (10-8 inches section down to the suggested 10 foot roughly 4 inches and drop the upper end of the bend down slightly.
The Mini 8 made coax antenna should easily handle legal limit so far, no issues at little over 1KW here.
What I have noticed besides the broad bandwidth, seems to be possibly slightly less in receiver noise- Rain does not affect the tunning – and seems to play better so far then my standard Wire Inverted L, I’ll need more time to really test this but as an example
Don N8KYX on average I would receive him at around 10-15 over s9- he is now 20-25 over- and when I was running 700-800 watts on the old antenna I would be about 5 over to him Now at 80 watts I’m now 5 over slightly more and 20+ at the 800 watts or so. For local performance that is a HUGE improvement
A group I tend to jump in with near Ashville Tennessee noticed a stronger signal as well
Then at 6:50 PM Saturday 10/3 right after N8KYX and I got the antenna up I worked 3 Spain station on FT8 back to back with 45 watts normally I would not hear EU stations before sunset (I would hear them after sunset but not before) this could have been a fluke and just good timing and band conditions , but signals reported on both ends were the same -12/-13.
As far as I can tell, this should not be used as a multiband inverted L – I do not think the high impedance and voltage would take long to breakdown the antenna near its current maximum point. I could be wrong but am not willing to find out. I’ll keep those interested posted on more findings so far I don’t see a reason to switch back to the standard Wire made inverted L unless your needing multiband usage- that seems to be along with its probably a much less prone to weather breaking it in wind etc., the only reasons to use one. If you can make the supports good get the vertical section minimum of 25 feet and the horizontal section 25 feet or more and do not need multiband usage the coaxial L is most likely a better performer!
A drawing of the crossover box can be found here
drive.google.com/file/d/10LNcwPKW6Z68SOrP5eWrB6yJYK3hYWwW/view?usp=sharing
Outline drawing of antenna can be found here
drive.google.com/file/d/1lSe7jE4tu4SbMagKr2qz3J0SIn8409kN/view?usp=sharing
I scaled this off the post below-
Subject: Re: 160 meter "inverted L"
View: Complete Thread (2 articles)
Original Format
Newsgroups: rec.radio.amateur.antenna
Date: 1996/07/23
Adam, in the early 80's Coleman Rowland, W4TWW published an excellent
article in CQ on the Coaxial Inverted L For 160 meters. I have built the
antenna and found it to be excellent. In theory the antenna is 1/4 wave
long and is made of a 50 ohm coax section and a 300 ohm twin lead section.
The coax section is an electrical quarter wave (246/FMHz times the
velocity factor) and the 300 ohm twin lead makes up the rest of the
quarter wave.
In reality through trial and error Coleman, found that the total length of
the antenna should be 230/FMHz; and the coax section should be 162.5/FMHz.
For 1.850 MHz this works out to 88 feet for the coax and 37 feet for the
twin lead for a total length of 125 feet.
Short the center conductor to the shield of the coax, and connect this to
the twin lead. The twin lead is shorted at this connection point and also
shorted at the opposite end.
Feed it at the other end of the coax, using 50 ohm coax from your rig and
connect the center conductor of the coax from your rig to the shield of
the coax making the antenna; and the shield of the coax from the rig to
the center conductor of the coax making the antenna. The center conductor
of the antenna coax goes to ground with as many radials as possible.
Mount it as vertical as possible and pull the rest out as horizontal as
possible. If you can go up 88 feet it's good to get the entire coaxial
section vertical, but if you can't it will still work well.
The antenna is very broad and will be useable across the entire 160 meter
band without a tuner. However, it is still fundamentally a vertical so
you need as many radials as you can possibly get.
I later made a change to W4TWW's design which I would recommend. I simply
replaced the 300 ohm twin lead with another piece of 50 ohm coax. It's
much stronger and I saw no difference electrically.
Good luck, Merv