View Full Version : What is the best antenna feedline length?

VE3PCD

09-04-2011, 02:46 PM

I need some expert help...

I have an mfj-1775 dipole antenna for 40, 20, 15 meters and I am trying to get the most efficiency as I can so I did some research on coax feedline lengths and came up with 55.5 feet of RG8/U to be what I need, this is based on the formula: Length=(492*volFac of coax)/F(mhz). This length gives me some multiple of 1/2 wave length.

I am confused as I play around with other lengths of the same coax I really don't notice much difference in signal strength or noise, example the 39.5 feet of RG8/U seem even better than the 55.5 feet of the same coax.

So the question is this, what is the best length of coax that can be use to improve the performance of your antenna system, and I have I missed something here?

Should I used the shortest 1/2 wave length that I can user?

note: Though I can hear well, I am still have some troube being heard(at 100%)

73,

-Peter

As far as I am concerned, the best length of coaxial cable is the length that gets from the antenna to the rig.

Used lots of antennas through the years and never gave it any thought beyond that.

Never had any problems either.

Chances are that you may always hear some DX stations that cannot hear you. There are several possible explanations for that some of which may have more to do with the other station than yours.

K9STH

09-04-2011, 05:33 PM

When you move away from the "design" frequency the length is no longer a half-wave!

Basically, except for some very specific applications, there is no "magic" length of coaxial cable. Basically, the length from the antenna to the rig plus maybe a couple of feet to allow the rig to be moved is generally optimum!

The concept of having only specific lengths of coaxial cable is one of those "olde wyves tayles" left over from "CB" operation. When the antenna is properly "matched", then the only thing you have to worry about is the attenuation in the cable and on HF that is generally not a concern except for some really long runs of cable.

Glen, K9STH

KG4RUL

09-04-2011, 05:41 PM

As far as I am concerned, the best length of coaxial cable is the length that gets from the antenna to the rig......

I have found that usually works best. :D

VE3PCD

09-04-2011, 06:13 PM

Thanks for the feedback...

What about the theory that you need a 1/2 wave length or some multiple of, and what about taking the velocity factor of the coax into play, these don't mean anything?

The halfwave coax idea may have originated in the CB world where antenna theory = voodoo arts. May have something to do with getting a repeated SWR reading at the feedpoint when read at the radio...this only would be true for a very limited range of frequencies ie, one band.//

If your antenna is trimmed for near 50 ohms with not much reactance at the feed point, ANY length of 50 ohm coax will work, the shorter the length the lower the loss will be.

SO...the shortest length that reaches from the antenna to the radio is the BEST length.

For non-resonant antennas, this is a different case alltogether, the length of balanced line can be part of the Z matching system and can be critical.:eek:

K0RGR

09-04-2011, 07:22 PM

Thanks for the feedback...

What about the theory that you need a 1/2 wave length or some multiple of, and what about taking the velocity factor of the coax into play, these don't mean anything?

No, if the antenna is properly matched, the line length doesn't matter. If the SWR is 1:1 at the antenna, it will be 1:1 everywhere on the line.

Make your coax whatever convenient length you need. If this were not the case, you'd need different lengths of coax for each band.

M0GVZ

09-04-2011, 07:46 PM

So the question is this, what is the best length of coax that can be use to improve the performance of your antenna system, and I have I missed something here?

The shortest length to get from the transceiver to the antenna is the correct answer.

Should I used the shortest 1/2 wave length that I can user?

No. Only CBers and morons who buy "no groundplanes or counterpoise required" verticals other than vertical dipoles need to use co-ax in 1/4 or 1/2 wavelengths as those types of antenna use the feedline to make up the missing half.

W5DXP

09-04-2011, 11:48 PM

What about the theory that you need a 1/2 wave length or some multiple of, and what about taking the velocity factor of the coax into play, these don't mean anything?

If it is a 50 ohm antenna, for matching purposes, the 50 ohm coax length doesn't matter. What does matter is if the antenna feedpoint impedance is 50 ohms and the characteristic impedance of the transmission line is not 50 ohms.

For instance, 1/2WL is a good length to choose for 450 ohm ladder-line feeding a 130' (80m) dipole.

The SWR on a 50 ohm antenna being fed with 50 ohm coax is 1:1 so, for matching purposes, the feedline length and velocity factor doesn't matter.

The SWR on a 50 ohm antenna being fed with 450 ohm ladder-line is 9:1 so, for matching purposes, the feedline length and velocity factor does matter.

WA6OKL

09-05-2011, 05:23 PM

As long as the antenna is matched to the coax, the only effect of the length of the line is insertion loss. This is the attenuation of the signal due to the resistance in the cable. It is pretty small in the HF bands if you are using good coax. Make sure the connectors are tight and terminated properly and feel free to give yourself the freedom of a few extra feet of line!

K8ERV

09-05-2011, 07:32 PM

As far as I am concerned, the best length of coaxial cable is the length that gets from the antenna to the rig.

.

That's what Abe said, only slightly differently.

TOM K8ERV Montrose Colo

AJ4CM

09-05-2011, 08:00 PM

The theoretical "best" length would be zero. Solder the feedpoint of the dipole directly to the output stage of your radio, then hoist it into the air and run a long mic cable to your operating position.

It's a bit inconvenient to change frequencies, though... :)

K8ERV

09-05-2011, 08:05 PM

But think of the great birdhouse it would make!!

TOM K8ERV Montrose Colo

G0GQK

09-05-2011, 09:34 PM

I'm one of those morons with a Cushcraft R5 vertical and its got me to Australia and New Zealand, can't go any further, and my coax is the length required from the antenna to the SWR meter. Don't know how long that is, but it works

W5DXP

09-05-2011, 09:40 PM

Solder the feedpoint of the dipole directly to the output stage of your radio, ...

I've done that with an SG-500 amp and 12v marine battery located directly at the base of a vertical monopole.

K4KWH

09-06-2011, 09:30 PM

No, if the antenna is properly matched, the line length doesn't matter. If the SWR is 1:1 at the antenna, it will be 1:1 everywhere on the line.

Make your coax whatever convenient length you need. If this were not the case, you'd need different lengths of coax for each band.

I have used this very statement to debunk some of this CB Voodoo you see around on and off the web. To wit, the mobile screwdriver. After explaining how and what a screwdriver does, I then used their own "3 foot, 9 foot, 18 foot" nonsense against them! :D Stating that if that were true, then since antenna theory is the same for all radio, I would then have to have 8-10 coaxes, each cut to a specific length, some of them up to 120 feet long down to 8 feet long in my car or pickup. Yet hams operate from 3-30 MHZ, using ONE coax of no particular length with an SWR of 1.2 to 1 thru out ALL those bands! I ask them to explain how come that is if their 11 meter horse hockey is true---and you will get people who are absolutely convinced you MUST cut your coax to a certain length. To date, NO ONE has ever even come close to formulating a satisfactory rebuttal!!! :D The couple who tried offered the most pitiful, most implausible, and the most laughable explanations! :D:D

J

WA8FOZ

09-08-2011, 09:07 PM

Attributed to Abraham Lincoln:

"A man's legs should be long enough to reach the ground.";)