Dual Band Antennas

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bronc076

bronc076

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Is there some magic to a dual band antenna? I have a Yeasu FT2980 2M radio and when I bought it I got an antenna I cut to length for the frequency I use and an NMO base, I picked the middle of the 2m band, like 146MHz if I remember what I did. Now I'm looking at dual band radios (Icom 2730A) which will transmit on both 2M and 70cm. I'm looking at dual band antennas and I have not done the math regarding their length. But what frequency are they resonant to? Or is it a factor, clearly a piece of wire 3' long will be a fraction of a waive for several frequencies, but I just don't know enough about this stuff to understand how a dual band antenna works. Is there some magic happening in the base I don't know about? I've been looking around on the interenet but I can't seem to find an explanation. They all seem to be pretty short so I'm thinking I'm comprimising my 2M performance. Still researching but I thought I'd ask you guys. What if I just use my 2M antenna, will it just be crap for 70cm? Guess I need an SWR meter.

A full waive at 440MHz is (2ft. 1 - 17/32in. or 0.648 M)
3/8 wave at 146MHz is (2ft. 4 - 7/8in. or 0.733 M)

courtesy Amateur Vertical Antenna Calculator - http://www.csgnetwork.com/antennaevcalc.html

So it is just a comprimise that works everywhere but not well, or again, am I missing something?

Thanks
Rob
KJ7JDB
 
1911

1911

chupacabra
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So it is just a comprimise that works everywhere but not well, or again, am I missing something?

Bingo. No, you're not missing anything.

I have a Larsen dual-band antenna on my 40 that is 1/2-wave on 2m (with a bottom-loaded coil) and I don't remember what on 70cm; it works well enough on 2m and I hardly ever use 70cm, but when I've tried it, it worked there too. I bought it when I got my dual-band radio, but if I were doing it again I would probably just get the best 2m antenna I could since I rarely if ever use 70cm.
 
JLH

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A quarter wave at 146mhz will also show a decent match at 450 but the takeoff angle is high. Simple, small and rugged though.
 
Bunga

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Agree with chupacabra. I got a spectrum analyzer and was really shocked how bad (inefficient) a lot of dual band antennas are. Pick your main freq and get a matched antenna. Don't get me started on HT antennas.
 
Pskhaat

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Agree with chupacabra. I got a spectrum analyzer and was really shocked how bad (inefficient) a lot of dual band antennas are. Pick your main freq and get a matched antenna. Don't get me started on HT antennas.
This is one of the reasons we stopped trying to come up with a dual-band radome antenna. Like in Oz, lots of folks are running two single bands with a band pass switch with better performance.
 
DaveInDenver

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This is one of the reasons we stopped trying to come up with a dual-band radome antenna. Like in Oz, lots of folks are running two single bands with a band pass switch with better performance.
You pick your poison to some extent.

Say you're running a pair of monoband antennas that you combine using a diplexer. You have insertion loss and an extra layer of connectors.

If you use a switch instead of passive filters you can reduce some of the loss but then you can't work both bands simultaneously.

I have antennas done both ways, a plain Larsen dual band (I like the NMO2/70) and two monoband coupled with an MFJ-916. Both work. But then again I don't see any particular issue with my Larsen mounted on an NMO punched in the middle of my roof (this is not an insignificant detail). SWR sweep shows nothing concerning and the on-the-air performance meets expectations.

Honestly, there's more important things to worry about than this. Using a dual band from a major brand that doesn't make ridiculous claims is going to work fine. The dual or more band antennas that claim super gain and have several coils are probably going to disappoint, though.

But there's nothing wrong with the basic concept of an antenna covering multiple bands. Cellphones and WiFi devices rely on the principle of using collinear coils to make an antenna work on multiple bands.
 
Kofoed

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A dual band is 1/2 wave length on 2m and does not need a grounded mount
 
Pskhaat

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Say you're running a pair of monoband antennas that you combine using a diplexer. You have insertion loss and an extra layer of connectors.

Yup, there is absolutely loss through the switch/diplexer. As much as the attenuation in a dual-band antenna? Or course depends on the resonating bands for sure.

The dual or more band antennas that claim super gain and have several coils are probably going to disappoint, though.

The gain game is weird. We keep the inductor low (yes, with drawbacks on that) but don't coil along the radiating length.
 
DaveInDenver

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Yup, there is absolutely loss through the switch/diplexer. As much as the attenuation in a dual-band antenna? Or course depends on the resonating bands for sure.
There would be no attenuation unless something is broken or not tuned. Depending on how a diplexer is made the losses would be similar to those in a matching or phasing elements.

If a multiband whip isn't tuned then it will show attenuation across your frequencies of interest. That's a design or construction issue, not a technical or theoretical one. As you go from one to two to more bands of coverage your margin of error for getting everything right is smaller. But a simple dual band with a coil in the middle intended to make a loaded whip on 2m (perhaps 1/2λ) and collinear or phased elements (perhaps two 5/8λ) on 70cm is well understood.

The antenna itself usually has enough gain to offset loses and ideally is slightly positive. No SWR sweep can tell you this overall performance. It can only show network losses or lack of them and antenna impedance, which ideally matches your expected impedance.

The special gain case would be a 1/4λ monopole that is considered unity gain. Any matching losses could be viewed as mathematically attenuation. But this antenna is actually 2.15 dB of gain over an isotropic antenna, so has gain in the real world depending on your reference (2.15 dBi vs 0 dBd).
The gain game is weird. We keep the inductor low (yes, with drawbacks on that) but don't coil along the radiating length.
That's a simplification, but generally yes loading coils should be at the feedpoint or center of an antenna with capacitance at the end, if you wish to use it.
 

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