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subzali
04-09-2008, 09:01 AM
Will our homebrew antennas run into problems if we try to use the 70cm band? I had thought that the 2m and 70cm bands were harmonic frequencies so it would be okay, but the question was brought up so I thought I'd better check.

subzali
04-10-2008, 07:30 AM
Did I ask a stupid question or does nobody seriously know?

Tanglefoot
04-10-2008, 08:51 AM
I don't believe an antenna tuned for 2m will load up very well on 70cm. It's possible to build a dual-band antenna...you just start with a 2-meter antenna and then put in an inductive coil (trap) at the 70cm 1/4 wave (or 5/8 wave) distance. Then the antenna will deliver a good SWR on both bands.

You can always check the SWR on 70cm just for grins. You can borrow a meter if you don't have one. I have a simple one.

Eric

corsair23
04-10-2008, 12:00 PM
Matt,

I too was wondering this. My thought was no because of that formula used to determine antenna lengths for various frequencies and wavelengths (which I can't remember at the moment)...

But, I bet Nathaniel or Dave with chime in. I think Nathaniel may be a tad busy with a possible new addition to the family :thumb: and Dave is probably cramming for that Yoda test :D

DaveInDenver
04-10-2008, 01:36 PM
Matt,

I too was wondering this. My thought was no because of that formula used to determine antenna lengths for various frequencies and wavelengths (which I can't remember at the moment)...

But, I bet Nathaniel or Dave with chime in. I think Nathaniel may be a tad busy with a possible new addition to the family :thumb: and Dave is probably cramming for that Yoda test :D
Cramming. Heck yes. My brain (well, at least the backs of my eyes) actually hurts.

Keep in mind resonance and wavelengths, it's not really fundamental frequencies and harmonics. An antenna must be at or close to electrical resonance to radiate electromagnetic energy. A length of wire will only resonate at certain lengths for a given wavelength, this is where a standing wave has set up. You get resonance either by being the right length (some multiple of 1/4λ) or including parts to make the antenna whip length look electrically like the right length. You can make a paper clip into a HF antenna with the right matching network and the radio will not complain, but you will get very little energy radiated from the paper clip since it's a weird multiple of 1/4λ.

So in the case of a 2m antenna fed by 50 ohms, you need to have a feedpoint impedance of about 50 ohms to work well (it's max power transferred to the antenna and minimal energy reflected). It just happens that a 1/4λ wire will both be naturally resonant and have a sufficiently low impedance to work (about 37 ohms). This phenomenon happens periodically as you go up in frequency (down in wavelength) when the antenna length lines up to odd 1/4λ steps (1/4λ, 3/4λ, 1.25λ, 1.75λ, etc.). So a 1/4λ 2m antenna can work at 67cm, 40cm, 29cm, etc. As you increase the length of the antenna, it doesn't radiate the same patterns at these wavelengths, but it can work up to a point.

You may be asking about the even stops of 1/4λ (1/2λ, 1λ, 1.5λ, etc.). They will also radiate energy, but the voltage and current phasing is 90 degree opposite and the natural feedpoint impedance is not low, but very high (around 2000 ohms in practice). So you need a matching network to your 50 ohm coax and that makes the antenna specific to one frequency where the antenna and matching network are tuned. A LC network only resonates at one tuned frequency.

But remember, you guys did not use 1/4λ whips, but 5/8λ. This length is really a special case, since it's not technically a resonant length. It's acts like a 1/2λ antenna in terms of resonance, but why is really not important. Anyway, a 1/2λ or 5/8λ would by itself maybe work at different frequencies if it wasn't for the impedance. You have to use that pesky matching network and that makes the whip specific to one frequency.

RicardoJM
04-12-2008, 06:58 AM
But remember, you guys did not use 1/4λ whips, but 5/8λ. This length is really a special case, since it's not technically a resonant length. It's acts like a 1/2λ antenna in terms of resonance, but why is really not important. Anyway, a 1/2λ or 5/8λ would by itself maybe work at different frequencies if it wasn't for the impedance. You have to use that pesky matching network and that makes the whip specific to one frequency.

Ok, if I am understanding correctly our home brew antennas will not work for both 2m and 70cm as built. However, they could be use for both frequencies with the addition of an antenna tuner?

DaveInDenver
04-13-2008, 07:36 AM
Ok, if I am understanding correctly our home brew antennas will not work for both 2m and 70cm as built. However, they could be use for both frequencies with the addition of an antenna tuner?
Maybe yes would be the correct answer, but with a stipulation.

What an antenna tuner does is match the antenna and radio, a little different in operation than what it sounds like. But it's not really 'tuning' the antenna itself. It's matching an antenna that would not naturally be resonant to your radio. But the antenna whip itself has natural characteristics and will only radiate as much energy as it's inherently going to do. Take that paper clip example, say it's 6" (0.15m) long unbent and you use an antenna tuner to match it to a 80m rig. The antenna tuner could (at least in theory, there is a practical limit) get system to look like it's resonating at 3.5MHz even though it's less than a 1/512th of a wavelength long. Essentially what will be happening is the matching network will be absorbing just about all of the energy and very little will be radiated. It would be more correct to say your dummy load is 'leaking' a little RF energy...

Basically, an antenna tuner helps you use anything that is remotely conductive as an antenna. But that does not mean that you will have a good antenna, we still have to obey the laws of physics. There is really no substitute for having the right length of wire in terms of performance.

With that said, it might be that with an antenna tuner the DIY antennas work reasonably. I dunno much about how commercial antenna tuners work, if they can add inductance and capacitance and how wide of a range they have. But it's possible that the box would essentially tune the matching coil at the bottom and if that is the case, then the antenna could radiate significant energy since you're not asking for the impossible. It would have some losses due to the additional components (additional coils, caps, etc. aren't without some compromise) and it would be something like 1.75λ for 70cm, so it would have some weird lobes in its pattern. But if someone had a tuner and an antenna analyzer, it would be a good experiment.

What to keep in mind here is that a low SWR at the radio only means you are not reflecting energy, it does not guarantee that all that energy is being radiated. A dummy load (which for newbies is a monster 50 ohm resistor that you use to tune and check your radio without interfering with others) has a perfect 1:1 VSWR, but radiates exactly zero RF energy. An antenna tuner only tries to make the SWR low, it cannot change the actual characteristics of the antenna.

Kiowa Kate
04-13-2008, 07:55 AM
If there is very little action on 70cm, isn't this a moot point?

Tanglefoot
04-13-2008, 06:28 PM
Here is my extreme high-performance base station antenna with its perfectly-tuned ground plane swamp cooler.

It's a 1/4 wave on 2-meters with a simple inductive trap at the 70cm 1/4 distance from the base. I'm afraid I don't know the properties of an effective trap, but a few spirals seems to do the trick!
This is an MFJ mag mount, BTW.

8646

subzali
04-13-2008, 08:21 PM
If there is very little action on 70cm, isn't this a moot point?

As Bruce said, we gotta chatter up the bands in order to keep them for Amateur use! :thumb:

The question came up because I have the dual band capable Yaesu FT-7800, so before trying to transmit on 70cm I thought it would be a prudent question to ask. Thanks for the help y'all!

DaveInDenver
04-14-2008, 05:39 AM
It's a 1/4 wave on 2-meters with a simple inductive trap at the 70cm 1/4 distance from the base. I'm afraid I don't know the properties of an effective trap, but a few spirals seems to do the trick!
The MFJ-1724B. Looks similar to the antenna I use for around town and on trails with low trees, a Larsen NMO2/70SH. It's 1/4λ on 2m, coil a few inches up and center-loaded 3/4λ on 70cm, overall about 19" tall. I would have guessed that was the same. But if it's 1/4λ on 70cm, remember that's about 7" and so the coil is probably spaced about that far up.