Ideas wanted for airband antenna 118-137 MHz. SWR < 2:1

I'm looking to try to make an antenna to cover the range 118-137 MHz. The constraints are basically

diameter < 20 mm.
Length, certainly less than 700 mm, but even that might not be possible. I think something in the range 300-500 mm is more likely to be acceptable
SWR < 2:1.
Power handling 20 W.
Fed at the base - it can't be centre fed.
Work without a ground-plane. (I might be able to get that restriction relaxed, but I don't think so).
Gain - not specified.

I can't think of anything that will meet this - short of including an attenuator to improve the return loss!

Can anyone else think of any sort of antenna which will have this bandwidth, at a decent return loss? If you can, please let me know.

Dave

the standard/most common antenna for that band is a sleeve balun dipole. Thats assuming you are using it as a base antenna and not on an aircraft.

for 20W power feed you are not going to be able to skimp in size etc as you would end up wasting the power put into the antenna by poor efficiency.
you will see these on airdromes far and wide across the world




Keeping low SWR across that bandwidth on TX will be difficult. its a pretty big ask. Keep the radiator as thick as possible. Even using aluminium / copper
tube will be an improvement over a wire. As this will increase the bandwidth considerably. But 20MHz you are going to be really pushing the envelope.
normally used across just a few megs 118 - 120MHz, 120 - 124 MHz etc.
In reality do you really need to TX across that whole 20MHz ? after all 135 - 137 is for weather satellite RX only. The antenna will receive across 118-137MHz bandwidth happily
I suspect you really only need a good SWR for TX from ~ 118 - 122MHz

lets see what others say

I will let you worry about mounting considerations

cheers
Dave

That's the spec someone has requested. I'll check on the weather (excuse the pun) 135-137 MHz is needed. From what you say it is probably not.

The antenna you show will be too long anyway, as there's no way I can use something 1 m long. I was thinking of the same design, but trying to electrically load it with a continuous inductor. But I know that tends to reduce bandwidth, when I want to increase it.

I think someone is asking the impossible, but I'm open to suggestions. Perhaps there's some magic antenna I don't know about!

Dave

I can make an antenna at any length and with low VSWR as long as required efficiency is in parity with antenna length relative to lambda.
Not too long ago did I make a bottom feed folded dipole with total length lambda/8 and bandwidth in similar range, 60% efficiency, but that was for 2.4GHz
At antenna feed, in bottom of antenna, was a matching network designed and a 100 mm long coax did connect the antenna with a radio.
In your case had I maybe tried a unfolded dipole, at least 100 nH bottom coil for upper element and a ferrite core in bottom of antenna for avoiding ground current, and a conventional matching network, also at bottom of antenna. For both antenna elements, use outer braid of a thick coax as it reduces losses.

E Kafeman said:

I can make an antenna at any length and with low VSWR as long as required efficiency is in parity with antenna length relative to lambda.

Click to expand...

I'd be interested in know what you consider the limits of what's possible in this case. To me the problem is the bandwidth. Getting a low SWR at a spot frequency is not hard, but getting it over a wide bandwidth is much more difficult - perhaps even impossible.

E Kafeman said:

I
Not too long ago did I make a bottom feed folded dipole with total length lambda/8 and bandwidth in similar range, 60% efficiency, but that was for 2.4GHz
At antenna feed, in bottom of antenna, was a matching network designed and a 100 mm long coax did connect the antenna with a radio.
In your case had I maybe tried a unfolded dipole, at least 100 nH bottom coil for upper element and a ferrite core in bottom of antenna for avoiding ground current, and a conventional matching network, also at bottom of antenna. For both antenna elements, use outer braid of a thick coax as it reduces losses.

Click to expand...

But what sort of "conventional matching network" can match such an impedance over such a large percentage bandwidth?

I can understand how you can match an antenna of lambda/8. In theory you can make that 100% efficient, if you built it with superconductors. But I don't see how you can achieve a decent return loss over a wide range of frequencies.

Making the elements wide does of course reduce losses as you say, but it also increases bandwidth. But I don't see how it's possible to match the impedance over a wide range like I've been asked to do.

Dave

Defining bandwidth in terms of VSWR do not say so much about actual antenna function.
A normal standard fullsize wire-dipole with VSWR 1:2 or less have a typical bw 10% of center-frequency.
Assume that you requried an antenna with a bw in terms of VSWR better then 1:2. Sure I would design an antenna with an 2 dB resistive attenuator as matching element. Any antenna would then have bw over several octaves. For low VSWR over a wide frequency range is a 50 resistor enough.

You ask for what matching network that can handle a such a large bandwidth. Actual bw in your case is it 15% of centerfreq.
My job would be easy if not more was required. A normal cellular phone embedded antenna today, should handle 1710-2170 MHz, with margins for detuning.
That is about 20% bw of centerfreq. Same antenna should also handle GSM850 and GSM900. Most of my customers do however not care to much about VSWR. They are more interested in TIS, TRP and efficiency and how well I can make the antenna to fit their chipset and a hardware with a very low antenna height and minimal antenna volume. It becomes in the end a very complex requirement so today must you work in parallel with antenna and matching network. TIS and TRP is a kind of VSWR, multiplied with antenna efficiency but for a complex impedance that changes over frequency.

But I don't see how you can achieve a decent return loss over a wide range of frequencies.

Click to expand...

You are welcome any day to visit me. By the way, with in a short future will cell-phone antennas be even more demanding, when full LTE specfication should be implemented. Continuous coverage 700-2700 MHz, by two antennas, each antenna very effective, each antenna blind for the other antenna (10 dB or better).

There are a number of ways of making microstrip antennas fairly wide band, and also of tuning them to have a decent return loss at a couple of frequencies, where each frequency can be changed, with minimal effect on the other. But none of these techniques are suitable for an antenna with the design limitations I have.

Dave

According to antenna theory, there's an inviolable limit for the bandwidth of a lossless antenna of given size relative to the wavelength.

The other point is, that without any ground plane, you obviously need to shift the feeding point somehow towards the center of the antenna rod, e.g. by an embedded coaxial cable, or a construct like the said sleve balun. It can be assumed, that a shortened dipole with a kind of continuous elongation means like a helical coil would be the best solution.

Your requirement, VSWR 1:2 for 15% of center-freq. for a bottom feed 50% shortened dipole is not too hard to design. In my design mentioned above was it for 2.4 GHz and 25% of full dipole length but it is more a scaling problem.
The main limitation is that high efficiency costs efficiency bandwidth for a given antenna type and volume, if design otherwise is optimized, as mentioned by FvM. This is not a limitation of resulting VSWR.

Classic things to optimize is to add bottom coil and top cap to improve efficiency of a shortened dipole, which can be done more or less effective for best compromise between efficiency bw and efficiency peak value. Making antenna mechanical unsymmetrical is a common way to make it better electrical symmetrical due to environment effects as often one monopole-part of antenna is much more effected by ground. Using ferrite and ceramic material in a wide-band, low volume dipole antenna, can be a help, such as if space for a top cap is limited or a secondary resonance is needed.
An example of top cap and bottom coil and a impedance transformer when a real short and compact dipole is needed:


Without impedance matching circuit are these variables much more limited, often resulting in a less optimized construction. It is a minor problem if an effective antenna have a weird feeding impedance over its whole bandwidth as long as a basic impedance network can correct it to a impedance that fits the radio (low VSWR). A conventional basic network is a PI or T network, discrete or as stubs. Sometimes can either type of net be selected, and then do I select that solution that results i less radiation of harmonics. Avoid:
1. Too extreme impedance transformation ratio as it require very ideal components and critical component tolerances.
2. Too steep antenna impedance variation over frequency, requires unpractical many links in a impedance correction network, which must be equal steep as the antenna impedance that you want to correct, but in opposite direction. I have as thumb of rule that each discrete component costs 0.4 dB. If the component not add antenna improvements better then this, find another solution.