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 Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

11-10-2018, 12:33 AM
 Member Join Date: Feb 2017 Posts: 179
Dipole design question

The darn internet is giving me a headache.. But I'm learning about velocity factor and how it affects things.

I'm building a dipole antenna for 137 Mhz and need to clear a few things up.

1st.. There are several instructions on the internet.. both are trying to build a double cross dipole to capture the signals coming from the NOAA satellites.. This happens at about 137.1 Mhz to 137.8 Mhz.. but lets just go with the 137 for simplicity.

The instructions on this page tell me my dipole(s) should be 38.25 inches long:
https://www.qsl.net/py4zbz/DCA.pdf

Then the instructions on this page tell me my dipole(s) should be 43.3 inches long (1100mm)
https://sourceforge.isae.fr/projects..._and_design/17

After a bit of reading, I followed along with the math on the sourceforge link and found indeed that the 43.3 inches seems correct.. but I don't know enough about this to know if the math is indeed correct or not.. Seems like it is though from my other dipole reading.

The only thing that these folks seem to agree on is the 21.5 inch separation between the dipoles. That's seems odd.

So what am I missing here?

I'm going to be using some 1/2 inch aluminum pipe for my dipoles. Each pipe will be screwed into a 1/2 inch threaded PVC tee at the center and I plan on drilling and tapping a hole just behind the threads to accept the coax connection point.

My first instinct is to cut the dipole length to 43.3 and cut it down if needed.. but why the difference in values from these two pages?
11-10-2018, 5:04 AM
 Member Join Date: Jun 2006 Posts: 6,877

In free space with thin wire, a dipole for 137MHz would be about 40.95 inches long. Make it from 1/2" pipe and it will be shorter. Nestle it close and in between some other metal and it will be a little shorter still due to the impedance being pulled down and the elements shortened to compensate for a 50 ohm match. The only real way to accomplish your goal is to hand tune the dipoles with a 50 ohm feedline under the conditions it will be used, like in a group of four spaced 21.5" apart.

I suspect the author in your first link is the actual designer of this antenna and I would trust his measurements as he probably has made more than one of these. If you make the elements from pipe you have to take the length of the coax pigtails into consideration as they are part of the antenna, so the pipe lengths will be shortened by the length of the pigtails. This might be a good time to invest in an antenna analyzer!

Quote:
11-10-2018, 7:06 AM
 Member Amateur Radio Join Date: Aug 2003 Location: Cincinnati, OH Posts: 204

If you’re this worried about making the dipole, there’s also a QFH antenna that works for this. The only thing with that is it has to be made in a certain direction to compensate for the natural spin of the satellite.
11-10-2018, 2:35 PM
 Member Premium Subscriber Join Date: Nov 2008 Location: Campbell County, Wyoming Posts: 424

For a dipole, the common formula is 468/(frequency in Mhz)=length in feet. This formula assumes a velocity factor of about 95% which is common for many conductors used for antenna construction. This formula also assumes the 'gap' in the middle of the dipole is of negligible length.

Many people use the same formula but for a 1/4 wave (234/F) for each element and then add the 'gap'. As both equations assume the antenna is in free space and multple wavelengths form anything else, it is suggested to add about 10% to the result and 'trim' the antenna with a SWR meter or antenna analyzer -which I would also recommend. Using an analyzer will get you 'right on'. If one is not available, you will likely not notice any difference in received signal quality by just cutting to either formula.

'Roll your own' can be fun as well as making the hobby more affordable, enjoy. If you ask around at your local amateur (Ham) club meeting, you likely can barrow an analyzer as once your project is done, you won't need it. If you can't don't worry, as stated above you likely could not detect any difference in reception before and after 'trimming'

Have fun
11-10-2018, 6:21 PM
 Member Join Date: Feb 2017 Posts: 179

Thanks for the info guys... I think I'm going to cut it a bit long as I can always trim the aluminum pipe back with a simple pipe cutter.

The antenna elements may be aluminum but I'm using a length of RG58a/u that has a velocity factor of .66

Going to have to play with the math I barely understand to see how that affects things. Two of the four dipoles must have coax leads that are a 1/4 wave shorter. So with that in mind, the velocity factor comes into play as I understand it.

Then, this whole antenna with the short RG58 coax lead will be connected to a 100 feet of RG8/U and run to an SDR dongle.
11-10-2018, 6:37 PM
 Member Join Date: Mar 2005 Location: So Cali Posts: 2,959

Quote:
 Originally Posted by Murphy625 \snip but I'm using a length of RG58a/u that has a velocity factor of .66
The velocity factor of the coax cable is not part of the antenna element velocity factor. So you would not have to figure that in for the antenna.
11-11-2018, 2:12 PM
 Member Shack photos Join Date: Dec 2008 Location: New Zealand Posts: 2,888

Quote:
 The velocity factor of the coax cable is not part of the antenna element velocity factor.
Yes, but it is part of the phasing harness so is definitely part of the final system.
__________________
Cheers - Martin ZL2MC - Palmerston North
11-11-2018, 3:36 PM
 Member Join Date: Feb 2017 Posts: 179

Quote:
 Originally Posted by cmdrwill The velocity factor of the coax cable is not part of the antenna element velocity factor. So you would not have to figure that in for the antenna.
Quote:
 Originally Posted by majoco Yes, but it is part of the phasing harness so is definitely part of the final system.
Thanks for confusing that heck out of me guys..

Ok.. so let me see if I have this straight.. The velocity factor of the aluminum antenna elements doesn't matter because all 4 of the dipoles are using the same aluminum and the signal is going to be whatever it is when it leaves the aluminum and heads into the coax.

Now the tricky part:
Because the 4 elements are connected to two coax leads, and those coax leads must be cut so one is a 1/4 wavelength longer than the other, the velocity factor comes into play because, given a specific frequency, the 1/4 wavelength at a velocity factor of .95 is going to be very different than a 1/4 wavelength at a velocity factor of .66.

Did I get that right or am I still screwed up?
11-11-2018, 4:35 PM
 Member Join Date: Jun 2006 Posts: 6,877

I think your close. Antenna elements will be affected by the diameter and if they are insulated or not. Larger diameter will cause them to be shorter for a particular resonant frequency and insulation comes under the velocity factor topic where it slows down the RF through the insulation and different types and thickness of insulation will affect the resonant frequency of the conductor beneath it.

If coax lengths are critical in your project you must know the velocity factor of the coax originally used and what you have so you can calculate any difference and make adjustments in lengths to make it work right. I would rather use the exact type and velocity factor coax specified in the project instructions.

A good antenna analyzer can also help you measure and determine exact 1/2 wave or 1/4 wavelengths for these projects. Its still not too late to get one!

Quote:
 Originally Posted by Murphy625 Thanks for confusing that heck out of me guys.. Ok.. so let me see if I have this straight.. The velocity factor of the aluminum antenna elements doesn't matter because all 4 of the dipoles are using the same aluminum and the signal is going to be whatever it is when it leaves the aluminum and heads into the coax. Now the tricky part: Because the 4 elements are connected to two coax leads, and those coax leads must be cut so one is a 1/4 wavelength longer than the other, the velocity factor comes into play because, given a specific frequency, the 1/4 wavelength at a velocity factor of .95 is going to be very different than a 1/4 wavelength at a velocity factor of .66. Did I get that right or am I still screwed up?
11-11-2018, 5:56 PM
 Member Join Date: Feb 2017 Posts: 179

Quote:
 Originally Posted by prcguy I think your close. Antenna elements will be affected by the diameter and if they are insulated or not. Larger diameter will cause them to be shorter for a particular resonant frequency and insulation comes under the velocity factor topic where it slows down the RF through the insulation and different types and thickness of insulation will affect the resonant frequency of the conductor beneath it. If coax lengths are critical in your project you must know the velocity factor of the coax originally used and what you have so you can calculate any difference and make adjustments in lengths to make it work right. I would rather use the exact type and velocity factor coax specified in the project instructions. A good antenna analyzer can also help you measure and determine exact 1/2 wave or 1/4 wavelengths for these projects. Its still not too late to get one!
Well that's part of my problem. The guys who designed the antennas tend to differ on the specifications.. and to make matters worse, neither of the designs are done in a way that would stand up to the elements for any appreciable length of time.
Apparently, they got the antenna design to work but didn't give any thoughts to mother nature, galvanic corrosion or UV (sunlight) degradation of materials.
In other words, their antennas will probably only work for short time frames measured in months before problems start to creep up.

I'm building something I can put up and have it last for a couple decades. Its not difficult, just have to use the right materials and put them together correctly.

Antenna elements made of aluminum, electrical grade PVC that will be lightly sandblasted and then painted with an outdoor paint, joints that are Teflon taped or otherwise sealed, and structure that can take high winds.

I just don't want to do it half way.
11-12-2018, 5:22 PM
 Member Premium Subscriber Join Date: Nov 2008 Location: Campbell County, Wyoming Posts: 424

I would say you are on the right track. When cutting the coax phasing sections, get as close as you can by calculation. The electrical 'length' of the phasing lines will not be effected much by external factors. The main issue is remembering that the coax 'length' is connection to connection, which may (or may not) be the same as overall length depending on how you 'hook things up' (wrapping around screws or adding connectors). In my own building, I attempt to get phasing lines as 'close' as practical, but realize 'perfect' is not going to happen. remember to 'cut' for the middle of the frequency range (in your case 137.6 to get 137.1, 137.62 and 137.9125). a 1/4 wave is different for each of these frequencies but using the midpoint means 'close enough'.

If you want another way to look at this, a wavelength at the frequency in question is 85.84 inches so an electrical 'degree' would be about 1/4 "-and 91 degrees is close enough to a quarter wavelength.