# 5Ghz antenna Using a Piece of Coax

#### electronewb

Joined Apr 24, 2012
260
Hi I need to replace 3 5Ghz (5.1-5.8Ghz) something similar to those ( http://www.pwsstore.com/images/products/detail/170558001.jpg )

But I need the flexible stuff and cannot find it anywhere I've been looking for a few days and I don't think anybody makes those in that range. Would using a piece of LMR100/200 (Rg174 won't stay up it would flex too much) with a SMA connector straight into the modem would do the trick?

#### Papabravo

Joined Feb 24, 2006
17,587
Coax with an SMA connector would actually shield the center conductor. Might not work as you expect. I'd use some piano wire on an SMA connector cut to the right length.

#### electronewb

Joined Apr 24, 2012
260
Coax with an SMA connector would actually shield the center conductor. Might not work as you expect. I'd use some piano wire on an SMA connector cut to the right length.
True about the shielding!!! Would you know the length? Would I need to make a choke at the bottom of the antenna with the right turns as well?

#### Papabravo

Joined Feb 24, 2006
17,587
Well the length could one of several choices, with the most popular being a quarter wavelength or 5/8 wavelength. A choke at the bottom will block all of the high frequency signal -- why would you want to do that? Not to mention the small detail that a lumped inductor at 5 GHz is very nearly a physical impossibility.

The frequency you use to calculate the length should be the geometric mean of the band edges. Wavelength is the speed of light divided by the frequency.

Before you do the calculation -- write down what you think the answer is. I think the result will surprise you.

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#### electronewb

Joined Apr 24, 2012
260
So that video is wrong?

#### Papabravo

Joined Feb 24, 2006
17,587
Nope, but the video is describing something different than what I thought you were trying to do. The construction in the video shows an end-fed dipole. I was talking about a simple end fed vertical such as the antennas that are used in portable phones and handi-talkies. The two turn open core inductor effectively shortens the antenna to a quarter wavelength.

Moving pictures are worth at least a couple of paragraphs.

#### Papabravo

Joined Feb 24, 2006
17,587
So what is the geometric mean of 5.1 GHz., and 5.8 GHz.?
What is a quarter wavelength at that frequency?

Are they consistent with the dimensions in the video?
What software was running in the video to check the antenna performance?

#### electronewb

Joined Apr 24, 2012
260
Would a 2.4Ghz antenna work for 5Ghz? I found the right type of antenna but it's only in 2.4Ghz

#### Papabravo

Joined Feb 24, 2006
17,587
As pointed out in the video -- it might work but it is not optimal.

#### Kermit2

Joined Feb 5, 2010
4,162
For a 1/4 wave antenna you are looking at 9/16 inch down to 1/2 inch.

Solder a copper washer on the end of your coax for a small ground plane and leave a half inch piece of the center conductor exposed above it.

You can stiffen it into proper shape using wood and electrical tape. Think popsicle sticks.

A true ghetto stub antenna.

#### electronewb

Joined Apr 24, 2012
260
Does the lenght on an antenna actually matters if you have more lenght that's needed?

Joined Nov 5, 2010
215
You can't hack around at 5GHz and get any kind of performance. Sure, some signal will get on and off the air but it will be severely attenuated.

And no, you cannot use a 2.4GHz antenna.

Yes, the length is critical. It has to resonate at frequency in order to have any gain.

Probably if you don't use the antennas the device was made for, you'll have a severe impedance mismatch and you'll be way off resonance so I really don't recommend doing this. Or just make little whips as simple as possible. Don't worry about "ground plane". You can tune them one by one by using a WiFi signal-strength utility, maybe in a phone. Start at about 1/8" longer than 1/4 wave and shave or clip little bits off the end until the signal strength maximizes.

Why do you need "flexible" antennas, or why wouldn't a rigid one would work?

#### electronewb

Joined Apr 24, 2012
260
For a 1/4 wave antenna you are looking at 9/16 inch down to 1/2 inch.

Solder a copper washer on the end of your coax for a small ground plane and leave a half inch piece of the center conductor exposed above it.

You can stiffen it into proper shape using wood and electrical tape. Think popsicle sticks.

A true ghetto stub antenna.
So you're saying that the total length for the antenna should be 9/16 of an inch to 1/2"?

#### Kermit2

Joined Feb 5, 2010
4,162
Yes

Joined Nov 5, 2010
215
I would start at 9/16" or even 5/8" and shave it down, checking signal strength with a cell-phone utility, until the signal strength maximizes.

If by chance the signal strength constantly decreases, start with a longer exposed whip of, like 3/4" and start trimming it down and checking signal strength until you can see it increase and hit a peak.

Keep wood and electrical tape and anything else well away from it. They will change the resonance of the antenna. Unless you do the shaving and signal maximizing with the wood and the tape in place.

But don't use electrical tape. Use heat shrink or something. Electrical tape has an awful adhesive that deteriorates into slime over time.