Dual Band Dipole 2m / 70cm Vertical Dipole Antenna with mast and flush mount.

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iMONITOR

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I ran across this antenna on Ebay yesterday and it caught my eye thinking a simple modification would make it suitable for a dual band VHF/UHF air-band antenna for scanners. However wouldn't the dipole elements conflict with each other being so close together? In any event it's interesting to note it was fabricated using 3D printing.

8723487235
 

nd5y

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However wouldn't the dipole elements conflict with each other being so close together?
It depends on the frequencies and how the elements are connected at the feedpoint.
Since you can't see inside the box there is no telling how that thing works.
 

iMONITOR

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It depends on the frequencies and how the elements are connected at the feedpoint.
Since you can't see inside the box there is no telling how that thing works.
He shows the frequencies in his add for using it as-is for amateur radio. VHF/UHF aircraft frequencies are not too far off, so it the lengths were changed/tuned for those bands so if the design works for amateur radio it should work for air-bands with regards to the dipole elements being so close to each other wouldn't you think? But I'm thinking it's not good for either.
 

nd5y

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People have made parallel dipoles for HF for decades. They can work ok but you have to mess with the element length and spacing.

If you have a dipole for one band and then another at 3x the frequency (like 2m and 70cm or 120 and 360 MHz) with a common feedpoint then the higher band will have a radiation pattern that is not perpendicular to the antenna and not useful when the antenna is mounted vertically.

There are better ways to make a 2m/70cm dual band dipole.

Something like that will not cover the entire 118-150.8 and 225-400 MHz air bands.
 

prcguy

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Yes, and to futher this, the antenna in question appears to have a 2m dipole and a 70cm dipole with a common feedpoint. As mentioned, a 2m dipole will have an ok match on 70cm but the radiation pattern will be skewed up and down wasting energy. If you simply place a 70cm dipole with the 2m dipole, on 70cm RF currents will flow on both dipoles because the 2m dipole is low impedance on 70cm and in effect you will be loading the feedpoint way down to perhaps 25-35 ohm range. I don't know how that antenna would work properly and not light up both dipoles on 70cm. Or maybe it does and they don't mention that....

A simple dipole made of thin elements will not have enough band width to adequately cover the entire 118 to 136MHz comm range of the VHF air band. Whats worse is the UHF air band is 175MHz wide and thin antenna elements are only going to be happy across a very small portion of that. UHF air band antennas usually have elements approaching 2" wide to give enough band width to work across the entire band.

Then you have the same problem as the 2m and 70cm antenna using the same feedpoint, the VHF air band X 3 equals the UHF air band, or in other words, a VHF air band antenna will be 3/4 wave resonant on UHF air and will have a low impedance at UHF and it will load down the feedpoint impedance of the UHF air band dipole. I have no recommendation on how to fix that.


People have made parallel dipoles for HF for decades. They can work ok but you have to mess with the element length and spacing.

If you have a dipole for one band and then another at 3x the frequency (like 2m and 70cm or 120 and 360 MHz) with a common feedpoint then the higher band will have a radiation pattern that is not perpendicular to the antenna and not useful when the antenna is mounted vertically.

There are better ways to make a 2m/70cm dual band dipole.

Something like that will not cover the entire 118-150.8 and 225-400 MHz air bands.
 

iMONITOR

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The side mounted dipole approach caught my attention as the top of my mast is already occupied.

Glad I didn't waste my money. I'm always looking for VHF/UHF air-band antenna solutions. So far I think the DPD Production MilTenna Omni seems to be the best solution, but again it too required top mounting unless you put it out a ways on a horizonal side mount away from the mast.
 

prcguy

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Anything you put on the side of the mast will interact and make it into a 2 element beam or create some lobes and nulls depending on the distance out from the mast. A full wavelength at VHF air band is a good 7 1/2ft and even that far from the mast it will not have a great omni pattern.

The side mounted dipole approach caught my attention as the top of my mast is already occupied.

Glad I didn't waste my money. I'm always looking for VHF/UHF air-band antenna solutions. So far I think the DPD Production MilTenna Omni seems to be the best solution, but again it too required top mounting unless you put it out a ways on a horizonal side mount away from the mast.
 

mancow

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Could it be a parasitic design with the shorter elements not electrically connected?
 

prcguy

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In the parasitic designs I think they feed the shorter 70cm element and the close coupling causes the longer 2m element to be a sympathetic radiator with a usable match on 2m. not sure how the dual dipole thing in this thread is constructed.


Could it be a parasitic design with the shorter elements not electrically connected?
 

nd5y

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Could it be a parasitic design with the shorter elements not electrically connected?
There is no way to tell without seeing inside the case.
I would think probably not. It's probably just Ebay garbage made by somebody that got the design from Youtube or something.

You can actually make a good 2m/70cm antenna that way by using a 2 m dipole and a close spaced 70 cm 1/2 wave element, or two 70 cm 1/2 wave elements one on each side of the 2 m dipole. The radiation pattern on 70 cm will be similar to 3 1/2 wave collinear array and will have around 3 dBd gain. Not many people know that.
 
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