Building Center-Fed Dipole for 39-45MHz Antenna

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kandrey89

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Hi,

I'd like to build my own RECEIVER ONLY center fed 1/2 wave dipole for 39-45MHz antenna. While I'm reading through quite a few resources, I thought I'd ask for pointed questions and get direct answers. I'm currently reading through Audio Systems Group, Inc. Publications and trying to figure things out, but your help is appreciated!

c=299.8e6 m/s
f=42e6 Hz

wavelength = c / f = 7.138 m
For 1/2 wave dipole, total length: 3.570m (1.785m per leg)
For 1/4 wave dipole: total length: 1.785m (0.892m per leg)
however to eliminate reactance of the antenna, it should be shortened slightly based on the velocity of electromagnetic wave in the material: Dipole antenna - Wikipedia
For a #10 AWG copper wire, velocity of propagation is about 0.95c, thus lengths are:
1/2 wave: total length: 3.390m (1.695m per leg)
1/4 wave: total length: 1.695m (0.848m per leg)

The larger the wire, the higher the SWR bandwidth. Cut the wire longer, so you could trim it in.

Since Antenna is balanced and coax is unbalanced, I need to use a Current BALUN to match the impedance of the antenna to the coax and also to prevent the coax from acting like part of the antenna.

Questions for receive only antenna:
  1. What Current BALUN configuration do I use for impedance matching and blocking coax from acting as part of the antenna?
  2. What core material # do I use, how many turns and using what cable/wire for the BALUN?
  3. What coax impedance cable do I use, 50ohm or 75ohm considering the scanner has 50ohm termination?
Thank you
 

RFI-EMI-GUY

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What I would suggest is to find a calculation for making one of those common 300 ohm twin lead FM FOLDED DIPOLE antennas and scaling it up for your center frequency of 42 MHz. It will probably be a bit more than twice the size of an 88-108 MHz dipole.

For a balun, simply use a cheap 75 to 300 OHM TV balun and use either 75 ohm or 50 ohm feedline to your receiver.

Now if you are going to use it for verticle polarization, you will need to physically move the feedline away from the dipole to prevent pattern distortion. Picture a capital "T" on its side.

If you are going to suspend it in free space with rope, the calculated measurements should be fine. If you are going to encase it in PVC , the electrical length will turn out longer and you will need to tune it for resonance by shortening it. You may be able to calculate the additional reduction in Vf, but without an analyzer you cannot be sure.
 
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kandrey89

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I bought Amazon.com: 75 to 300 Ohm UHF/VHF Matching Transformer: Electronics but I don't think it's working like it should, although perhaps my weird dipole I built long time ago is to blame, a 1.2m leg with 0.45m leg dipole made from copper pipe. But I did not notice that it was isolating the coax from the antenna properly.

Thanks for reminding me that in free space, the coax should run perpendicular away from the antenna.

Do you have any example links to what you mean by common 300ohm twin lead FM folded dipole?
Thanks
 

RFI-EMI-GUY

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Search for FM BROADCAST 300 OHM FOLDED DIPOLE CACULATOR. The older websites that are for BROADCAST enthusiasts are the best. I would do the calculations by hand because I am finding a lot of new java script calculators are based on AI and plainly wrong and innacurate.
 

RFI-EMI-GUY

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The antenna I describe consists of 300 OHM twinlead that is soldered at the ends and fed in the center by a piece of 300 OHM twinlead. If you ever bought an old FM stereo tuner it was the antenna that was included and you attached it to the wall with thumbtacks.
 

RFI-EMI-GUY

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That balun you got from Amazon appears to be crap. The description says "3 capacitor type". It is a crappy gimmick. It cannot be a balun with that design. Get one that has ferrite balun inside. Sometimes an old proven part from e bay is better than shiny cheap Chinese from Amazon.
 

ko6jw_2

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Any dipole has a low impedance at the center feed point. A 4:1 will not help. The problem is to match the balanced load the dipole to an unbalanced coaxial feed line. A 1:1 balun is a possible choice. You can also use several turns of coax as a choke. Do not use TV transformers.

Next, I am assuming the since you are in California, you are building this to receive the CHP.

The problem with vertical dipoles, as mentioned above, is that they need to have the feed line run a quarter wave length at right angles to the dipole and likewise the supporting structure should be a quarter wavelength away to avoid interaction. This makes vertical dipoles somewhat unwieldy and difficult to mount.

I would suggest a j-pole. Properly made it will have the same performance and is much easier to mount. Many people on these threads will put down j-poles and say they don't work etc. It's nonsense. Properly made and fed they will work well.

Don't sweat the measurements for a receive only antenna. Likewise, your scanner is only nominally 50 ohms. It will be quite happy with 75.

If you are interested in precision, get an antenna analyzer. Nobody ever made a good antenna from formulas alone. Too many other variables.

Arrow antennas will custom make ground planes for any VHF/UHF frequency. Telewave sells vertical dipoles for 42Mhz. Very expensive, but excellent performers.

Last, if you want to make DIY antennas, get an ARRL Antenna Book.
 

kandrey89

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Yes, I'm making a dipole for CHP reception.

ANT42D -Folded Dipole Antenna 38.5-47 MHz - Telewave, Inc looks nice, with a center freq of 42.75MHz, however $2300 damn... :D

A year or two back, I read a lot about current BALUNs and how to make them, but forgot it all. Was interested in spectrum analyzer with VSWR bridge but $1500 cannot be justified for my brief use/interest, neither would a $200 antenna meter with limited performance and use.
 

dlwtrunked

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Any dipole has a low impedance at the center feed point. A 4:1 will not help. The problem is to match the balanced load the dipole to an unbalanced coaxial feed line. A 1:1 balun is a possible choice. You can also use several turns of coax as a choke. Do not use TV transformers.
...

Folded dipoles have a 300 ohm (actually impedance. It is not just an issue about balanced vs. unbalanced.Sect. 2.4 at
Dipole antenna - Wikipedia
 

ko6jw_2

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The op wants to build wire antenna not a folded dipole. By the way, most 300 ohm twin lead is useless. If you want to build a folded dipole use ladder line.

Twin lead folded dipoles were great for making quick cheap and dirty FM and TV antennas and not much else. I've built many of them.
 

ko6jw_2

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Yes, I'm making a dipole for CHP reception.

ANT42D -Folded Dipole Antenna 38.5-47 MHz - Telewave, Inc looks nice, with a center freq of 42.75MHz, however $2300 damn... :D

A year or two back, I read a lot about current BALUNs and how to make them, but forgot it all. Was interested in spectrum analyzer with VSWR bridge but $1500 cannot be justified for my brief use/interest, neither would a $200 antenna meter with limited performance and use.

I understand. The SWR meter is of no use if you are not transmitting. Antenna analyzers are <$500. However, That's still as much or more than most people spend on a scanner.

The Arrow ground planes are relatively cheap and will be designed to your specs and tested. Telewave does offer discount to hams. We just put up a two bay vertical dipole array at one of our repeaters. Beautiful antennas.
 

ko6jw_2

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Additional thought for those wanting to make center fed antennas. Consider an extended double zepp. Fed with a tuned ladder line of about 1/8 wavelength and then coax with a choke. More gain than a half wave dipole. I knew a guy who made one for 11 meters. I myself made one for 6 meters. Excellent performer.
 

kandrey89

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Additional thought for those wanting to make center fed antennas. Consider an extended double zepp. Fed with a tuned ladder line of about 1/8 wavelength and then coax with a choke. More gain than a half wave dipole. I knew a guy who made one for 11 meters. I myself made one for 6 meters. Excellent performer.
Any links to guides, pictures and how it's assembled as well as tips, ie basically a cookbook, would be appreciated.
 

kandrey89

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Additional thought for those wanting to make center fed antennas. Consider an extended double zepp. Fed with a tuned ladder line of about 1/8 wavelength and then coax with a choke. More gain than a half wave dipole. I knew a guy who made one for 11 meters. I myself made one for 6 meters. Excellent performer.
That antenna appears to have a horizontal polarization and is too big at 30ft length: West Mountain Radio - Double Extended Zepp Antenna Calculator
 

majoco

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Strangely enough, I have just refurbished a dipole for 75MHz made out of old low band TV antenna parts. I certainly haven't designed it down to three places of decimals. Each leg is 1 metre long (300/75 = 4 metres =1 wavelength). I made a balun from the 300ohm to 75 ohm binocular core. 75 ohms to 50 ohms, 75/50 = 1.5, square root of 1.5 = 1.22 turns ratio, 1.2 is close enough. 1.2 to 1 = 3 to 2.5 turns on the balun core. Put the balun in a screw top pill box, coax in one end and two leads out the other to the vertical elements. Sealed the holes in the pill box with silicon sealant and ty-rapped it to the arm about 5cm away. Works a treat. 4 metres off the ground, picks up stations 50km away.
 

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Consider using aluminum tubing as your antenna is gonna be 11 feet in length. I made a 33MHz using the radials from an old RS Archer 20-014.

A few pics of the antenna and the 20-014
 

kandrey89

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Consider using aluminum tubing as your antenna is gonna be 11 feet in length. I made a 33MHz using the radials from an old RS Archer 20-014.

A few pics of the antenna and the 20-014
I thought the radiation pattern of a dipole is perpendicular to its length, shouldn't dipoles be vertically mounted, how come yours is horizontal?
 

RFI-EMI-GUY

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I thought the radiation pattern of a dipole is perpendicular to its length, shouldn't dipoles be vertically mounted, how come yours is horizontal?

The DIPOLE antenna should be vertically polarized for land mobile radio like CHP. ot sure why 217 chose horizontal for his setup.

The construction of a receiving antenna is not so critical as to require a VNA or VSWR meter to tune it. Using 72 OHM cable TV coax vs 50 OHM is not a problem. In fact at UHF frequencies 72 OHM cable has less loss. A ferrite transformer TV type BALUN will work fine for a folded dipole. Go ahead and build an antenna per calculations and be confident that is done per plan it will work fine.
 

RFI-EMI-GUY

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Additional thought for those wanting to make center fed antennas. Consider an extended double zepp. Fed with a tuned ladder line of about 1/8 wavelength and then coax with a choke. More gain than a half wave dipole. I knew a guy who made one for 11 meters. I myself made one for 6 meters. Excellent performer.
The OP wants an antenna for receiving CHP. He needs a vertical dipole antenna.
 
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