2m and 70cm yagi with single feed

[VK4WDM]

I have a 2m and 70cm mounted on a single boom that works quite well. ATM I have two separate feedlines but am considering getting a duplexer to reduce the weight of coax on the mast.

Now to my question: do I really need a duplexer? HF "fan dipoles" use a single feed line on the basis that the antenna that is resonant at the frequency of the applied RF power will radiate.

Does the same principle apply at VHF/UHF? Why not just feed both antennas with a single feedline without a duplexer?

73

Wayne VK4WDM

[VK4UH]

You may not be that Lucky Wayne.

With the HF antennas the non-resonant bits usually present a high impedance as so do not accept much of the power from the feed have little effect on the over all feed impedance. The resonant bits take the power and the rest effectively disappears.

With Yagis and their respective matching networks, the non-resonant partner often presents a low impedance and effectively "shorts out" the feeder disrupting the overall line impedance and disrupting power delivery to the resonant partner.

A nice idea though

Kevin VK4UH

[VK3YE]

Now to my question: do I really need a duplexer? HF "fan dipoles" use a single feed line on the basis that the antenna that is resonant at the frequency of the applied RF power will radiate.

Does the same principle apply at VHF/UHF? Why not just feed both antennas with a single feedline without a duplexer?

HF fan dipoles aren't always straightforward since there can be significant interaction between the elements.

144 & 432 is a similar harmonic relationship to 7 & 21 MHz. And 7 & 21 MHz on the one fan dipole isn't normally recommended since the 7 MHz dipole (being 3/2 wavelength) will radiate appreciable energy on 21 MHz.

This can be put to advantage when we use a 2m 1/4 groundplane on 432 or a 7 MHz dipole on 21 MHz.

Fine for casual operating but maybe not ideal if part of a beam.

However there must be other ways to get around it. It may be worth checking if a 144 MHz driven element can excite a 432 MHz driven element that is close but not electrically connected to the feedline or whether complications would remain (can't recall where I read about this).

EDIT: http://www.qsl.net/dk7zb/Duoband/10mdipole+3el6m.htm is a dual band driven element (nested inside a yagi for the higher band). However the frequency ratio is approx 2:1 so the longer element presents as a safely high impedence on the higher band.

EDIT 2: The thing I was thinking of was 'open sleeve'. http://www.wb5cxc.com/dualv_ant.html is a dual band / single feed 2m/70cm yagi like what you were seeking. Also see http://www.qsl.net/dk7zb/

[VK4TI]

I have a 2m and 70cm mounted on a single boom that works quite well. ATM I have two separate feedlines but am considering getting a duplexer to reduce the weight of coax on the mast.

Now to my question: do I really need a duplexer? HF "fan dipoles" use a single feed line on the basis that the antenna that is resonant at the frequency of the applied RF power will radiate.

Does the same principle apply at VHF/UHF? Why not just feed both antennas with a single feedline without a duplexer?

73

Wayne VK4WDM

Easy simple solution would be a relay ?

[VK4WDM]

Hi Barry

Yes a latching coaxial relay could be used, but a one with low loss at 432MHZ and capable of handling 100W would probably cost as much as a diplexer. I will cost out both options and see what I come up with.

I notice that there are 2m+70cm+23cm triplexers which might even be a more attractive option.

73

Wayne

[VK6OX]

Another solution Wayne, particularly if you wish to tx/rx both 2m and 70cm (almost) simultaneously, is to use two diplexers - one at the rig(s) end and one at the antenna.
This is the method I have used with my TS-2000 and 2 separate yagis. One feedline only.

[VK4APN]

I have a 2m and 70cm mounted on a single boom that works quite well. ATM I have two separate feedlines but am considering getting a duplexer to reduce the weight of coax on the mast.

Now to my question: do I really need a duplexer? HF "fan dipoles" use a single feed line on the basis that the antenna that is resonant at the frequency of the applied RF power will radiate.

Does the same principle apply at VHF/UHF? Why not just feed both antennas with a single feedline without a duplexer?

73

Wayne VK4WDM

I made this antenna up several years ago - worked surprisingly well. I used too heavy materials so rear mounting was just usable - lighter boom and elements if I made again.
Paul
Cairns

[VK2KPK]

I made this antenna up several years ago - worked surprisingly well. I used too heavy materials so rear mounting was just usable - lighter boom and elements if I made again.
Paul
Cairns

Lighter than that quoted in the .pdf?

Geoff.

[VK4WDM]

I must say that a log-periodic did slip under my radar. Certainly a candidate with quite good gain for size. I have a some 10mm wide, 2mm thick ali strip. I wonder if that could be used for the elements? (Thoughts anybody?).

73

Wayne VK4WDM

[VK4WDM]

Thanks for the info guys. I am now seriously thinking about an LPA to cover all my VHF/UHF needs and have started a new thread on that topic.

73

Wayne VK4WDM

[VK4APN]

I made this antenna up several years ago - worked surprisingly well. I used too heavy materials so rear mounting was just usable - lighter boom and elements if I made again.
Paul
Cairns

Lighter than that quoted in the .pdf?

Geoff.

The boom material I used was on hand and twice as thick as needed!!!! For rear mounting I had to fashion a stronger bracket - for horizontal mounted at COG no problem.

[VK2KPK]

Thank you.

Geoff.