Base antenna coax length

[Josh380]

So, on suggestion from a friend and ham operator, I'm building a basic dipole antenna which will go up in the attic of my house (HOA). There is no aluminum sheeting between the attic and the shingles. The shingles are not the heavy thick shingles, but more like the ones that look similar to sandpaper.

His suggested design involves this dipole "centre" here and two 102" steel whips. This should eliminate the need for tuning, making for a relatively easy install.

I have a great match on my truck..nearly flat from 26.5-28 MHz, 18 FT. of LMR 200 Coax. My base setup would involve LMR 400. I've heard 18 Ft. is typically the best coax length, and I know there are calculators out there for this stuff, how much further could I make my run should I need to, without going above SWR tolerances? Ideally the antenna will be mounted just above the radio in the attic, to allow for the shortest possible run.

For reference I'm runnning an RCI 2970N2, with an Astron 75A supply.

 

[prcguy]

9ft steel whips are expensive these days and with shipping you can easily pay $60 for just one. Since this is for your attic I think copper wire would be a much better and cheaper way to go. You don't even need a dipole "center", just split off the center conductor and braid and head the copper wire out in two different directions. $3 in parts and you can make a fine dipole.

On the coax length, there is no magic length. Use the least amount you need to reach the radio and use the lowest loss you can afford. I would also recommend using a ferrite type choke balun at the feed point to decouple the antenna from the coax. For about $10 you can get an FT-240-61 ferrite toroid and wrap 12 turns of your coax through it and have a fantastic performing choke for 27MHz.

So, on suggestion from a friend and ham operator, I'm building a basic dipole antenna which will go up in the attic of my house (HOA). There is no aluminum sheeting between the attic and the shingles. The shingles are not the heavy thick shingles, but more like the ones that look similar to sandpaper.

His suggested design involves this dipole "centre" here and two 102" steel whips. This should eliminate the need for tuning, making for a relatively easy install.

I have a great match on my truck..nearly flat from 26.5-28 MHz, 18 FT. of LMR 200 Coax. My base setup would involve LMR 400. I've heard 18 Ft. is typically the best coax length, and I know there are calculators out there for this stuff, how much further could I make my run should I need to, without going above SWR tolerances? Ideally the antenna will be mounted just above the radio in the attic, to allow for the shortest possible run.

For reference I'm runnning an RCI 2970N2, with an Astron 75A supply.

 

[mmckenna]

I have a great match on my truck..nearly flat from 26.5-28 MHz, 18 FT. of LMR 200 Coax. My base setup would involve LMR 400. I've heard 18 Ft. is typically the best coax length, and I know there are calculators out there for this stuff, how much further could I make my run should I need to, without going above SWR tolerances? Ideally the antenna will be mounted just above the radio in the attic, to allow for the shortest possible run.

The 'best' coax length is what ever is the shortest possible length between your radio and your antenna.

The "tuned lengths" of coax only serve to hide antenna tuning issues. They do not make the antenna work better. If your antenna is designed, installed and tuned correctly, the amount of coax between the radio and antenna is irrelevant. In fact, running more coax than necessary to connect the radio to the antenna results in more signal loss. That signal loss can be negligible on shorter runs and with better cable, but it can be significant on longer runs and cheaper cable.
And that loss works in both directions. It'll reduce the amount of power that makes it from your radio to your antenna, and it'll reduce the amount of received signal that makes it from the antenna to your radio.

His suggested design involves this dipole "centre" here and two 102" steel whips. This should eliminate the need for tuning, making for a relatively easy install.

102" on it's own may be too short. Usually those 102" whips were designed to use a 6" tall spring at the base. 108" is usually more likely to give you a better SWR on CB frequencies.
But wire is easier to tune. Difficult to cut stainless whips. Make it out of wire, keep them a few inches long, and trim for lowest SWR.

 

[Josh380]

9ft steel whips are expensive these days and with shipping you can easily pay $60 for just one. Since this is for your attic I think copper wire would be a much better and cheaper way to go. You don't even need a dipole "center", just split off the center conductor and braid and head the copper wire out in two different directions. $3 in parts and you can make a fine dipole.

On the coax length, there is no magic length. Use the least amount you need to reach the radio and use the lowest loss you can afford. I would also recommend using a ferrite type choke balun at the feed point to decouple the antenna from the coax.

I actually picked up two 102" whips from a CB shop semi-local to me, for $35 each. Dipole "centre" already ordered and should be 50 Ohms. No balun needed. Since this will be a T Dipole, I imagine the antenna will be much lower than the peak of the roof. I don't imagine I'll need more than 18ft.

Mmckenna:

I've heard of using springs..I didn't use one on my truck and my friends antenna analyzer has me at 1.2 between the above frequencies. Not bad IMO. If using a spring will get me lower than that, then I might consider it.

Unfortunately, everything has already been purchased. I've had quite a bit of LMR 400 laying around for awhile, so I plan on putting it back into service.

 

[mmckenna]

Mmckenna:

I've heard of using springs..I didn't use one on my truck and my friends antenna analyzer has me at 1.2 between the above frequencies. Not bad IMO. If using a spring will get me lower than that, then I might consider it.

Unfortunately, everything has already been purchased. I've had quite a bit of LMR 400 laying around for awhile, so I plan on putting it back into service.

If you are using a tuned length of coax between your CB and antenna, as in the 18' you mentioned, any SWR reading done from the radio end is just going to show the cumulative effects of the antenna + the possible effects of the coax. Try it again with a random length of coax and see what it shows. Might still show 1.2:1, or may likely show something different. Ideally you want the antenna tuned for lowest SWR, not the coax.

 

[Josh380]

If you are using a tuned length of coax between your CB and antenna, as in the 18' you mentioned, any SWR reading done from the radio end is just going to show the cumulative effects of the antenna + the possible effects of the coax. Try it again with a random length of coax and see what it shows. Might still show 1.2:1, or may likely show something different. Ideally you want the antenna tuned for lowest SWR, not the coax.

Ok. Good advice. Thanks.

I have a NanoVNA. Still trying to learn how to use it correctly, though. Seems kind of difficult.

 

[mmckenna]

Ok. Good advice. Thanks.

I have a NanoVNA. Still trying to learn how to use it correctly, though. Seems kind of difficult.

Yeah, I've got one too. I also have access to high end stuff at work. Even though I've been using the commercial stuff for years and understand it pretty well, the NanoVNA's took some work to learn. Still not comfortable with them. There's some good tutorials on The Youtubes, and some not so good ones.

 

[prcguy]

A balun is not needed but its a really good idea. You are building an unbalanced dipole antenna where one of the elements connects directly to the coax shield. The coax shield can and probably will be part of the antenna and it can radiate and also bring noise up the coax from other things in the house introducing interference to your reception. A choke balun will isolate the coax shield from the antenna and prevent hot coax and other problems. Its a just a good way to do business.

`

I actually picked up two 102" whips from a CB shop semi-local to me, for $35 each. Dipole "centre" already ordered and should be 50 Ohms. No balun needed. Since this will be a T Dipole, I imagine the antenna will be much lower than the peak of the roof. I don't imagine I'll need more than 18ft.

 

[Josh380]

A balun is not needed but its a really good idea. You are building an unbalanced dipole antenna where one of the elements connects directly to the coax shield. The coax shield can and probably will be part of the antenna and it can radiate and also bring noise up the coax from other things in the house introducing interference to your reception. A choke balun will isolate the coax shield from the antenna and prevent hot coax and other problems. Its a just a good way to do business.

`

Ok thanks. Ordered an MFJ-915. So this is to be connected at the antenna, correct?

 

[prcguy]

Yes at the antenna. The $10 ferrite core I mentioned earlier will work much better than the MFJ-915 for CB use. At best the MFJ is good for about 20dB isolation where you should get over 30dB with 12 turns of coax through an FT-240-61 core. Great performance is not about $$, its about knowing the best practice.

Ok thanks. Ordered an MFJ-915. So this is to be connected at the antenna, correct?

 

[Josh380]

Yes at the antenna. The $10 ferrite core I mentioned earlier will work much better than the MFJ-915 for CB use. At best the MFJ is good for about 20dB isolation where you should get over 30dB with 12 turns of coax through an FT-240-61 core. Great performance is not about $$, its about knowing the best practice.

Yeah but 12 turns with LMR 400 is not my idea of fun. Using what I have to work with to a degree.

 

[StaticDischarge]

You are building an unbalanced dipole antenna

Close... a diapole = two identical length radiators = Balanced (BAL in Balun) to coax which happens to be Unbalanced (UN in Balun) so therefore a balun is used to marry a balanced load to a unbalanced source and vice versa...

Yeah but 12 turns with LMR 400 is not my idea of fun

You could go with RG8x or equivalent to make the turns then barrel connector into your LMR400...

 

[prcguy]

$10 ferrite core, 10ft of LMR240 or RG-142 coax, a male and female connector and for maybe $20 you have a choke that is vastly superior to the MFJ.

And the best part is when one of your friends is in the same position needing a choke balun, you can say, "a $10 ferrite core, 10ft of coax and a couple of connectors, maybe $20 tops and your done. Been there, done that...."

Yeah but 12 turns with LMR 400 is not my idea of fun. Using what I have to work with to a degree.

 

[FPR1981]

9ft steel whips are expensive these days and with shipping you can easily pay $60 for just one. Since this is for your attic I think copper wire would be a much better and cheaper way to go. You don't even need a dipole "center", just split off the center conductor and braid and head the copper wire out in two different directions. $3 in parts and you can make a fine dipole.

On the coax length, there is no magic length. Use the least amount you need to reach the radio and use the lowest loss you can afford. I would also recommend using a ferrite type choke balun at the feed point to decouple the antenna from the coax. For about $10 you can get an FT-240-61 ferrite toroid and wrap 12 turns of your coax through it and have a fantastic performing choke for 27MHz.

I agree with this. I build kickass dipole antennas using dual 16 gauge copper lamp cord from the local hardware store. You're wasting your money using steel. That's cost prohibitive IMHO. And I agree with PRC. I build 2 to 3 dipoles a week for people and do not use a "centre." I usually do a direct wire solder job, coat it with hot glue, wrap it with electrical tape and be done with it. Works great.

 

[prcguy]

Indoors you can get away with almost anything, no weatherproofing to worry about, bare connections ok, no wind to wiggle things, etc.

I agree with this. I build kickass dipole antennas using dual 16 gauge copper lamp cord from the local hardware store. You're wasting your money using steel. That's cost prohibitive IMHO. And I agree with PRC. I build 2 to 3 dipoles a week for people and do not use a "centre." I usually do a direct wire solder job, coat it with hot glue, wrap it with electrical tape and be done with it. Works great.

 

[FPR1981]

Indoors you can get away with almost anything, no weatherproofing to worry about, bare connections ok, no wind to wiggle things, etc.

This one I built out of dual 18 gauge speaker wire and featured in an earlier video. It does have a center. It was from a Workman BS-1 dipole. But I checked the SWR last night after months in an Ohio winter, and she's holding very steady.

This was the video of the antenna build/install