[SOLVED] Improving FM transmission 12dBm on 20meters

[kripacharya]

I have capability of putting out 12.5 dBm on 20 meter band. My Tx antenna is terrible - a coat hangar variety. i.e. a 'short' whip.

(a) What sort of receiver sensitivity do I need to pick up this signal at 100 mtrs ?

(b) how do I interface/ match this to the antenna (Tx side) for best performance ?

 

[biff44]

20 meter band, so an "efficient" antenna will be 5 meters long. Anything shorter will end up not being as efficient a radiator, despite if you match its impedance of not. Try improving the ground plane, which is just as important as the antenna whip. Try making it longer, You could do a matching network. If it were me, and I needed the small size, I would live with the antenna and just drive it with a 1 watt amplifier. Plenty of chips around around for like $5.

 

[betwixt]

At 100m distance you probably don't need an antenna at all. I can reach most of Europe on 20m band with a short wire antenna.

Brian.

 

[vfone]

You might reach ionosphere bounce with a wire, but you need at least 1/2 Watt for doing this (and good propagation).
12dBm is much less than this, and even 100m could be hard to reach without some resonant antenna. For saving space I would recommend a small loop antenna (~1m diameter) and a resonant LC network for matching.

 

[betwixt]

I disagree, 100 miles might need 1/2 Watt and a good antenna but this only has to cover 100 metres so attempting ionospheric bounce would be dramatic overkill.

Brian.

 

[vfone]

I never argue on the net, but 12dBm TX power WITHOUT a resonant antenna and a good receiver at the other end, would not make a link in 20m.
Might work on higher frequencies as VHF and UHF, but not on 14 MHz.
This remember me when years ago I was talking with a friend X in 20m (14.200), and I told him that I know that another friend Y is listening us on the same frequency.
He said: How do you know?
I said: He is using TS-950 which has first IF on 73.05 MHz and the heterodyne receiver is doing high LO injection, so the first LO is now on 87.25 MHz, and I can hear the leakage of the signal if I tune my receiver on that frequency.
Of course I lied, but was a good one

 

[betwixt]

Unfortunately my sig genny and my 20m TX are packed away at the moment while I rebuid my house or I would do tests. I'm pretty sure it would work though, unless the antenna was specifically tuned to reject the frequency being used. The obvious solution is to use semaphore or shout loudly over that distance though :smile:

Brian.
aka GW6BWX

 

[davenn]

All this really begs the questions ....
Why does the OP only want to go 100m using the 20M ham band ?
Is he a licenced Amateur operator ?

If he is, it would be much easier to use a 2M or 70cm freq to get a signal across a short distance like that

cheers
Dave
aka VK2TDN

 

[volker@muehlhaus]

Why does the OP only want to go 100m using the 20M ham band ?

There's an ISM band at 13.56 MHz.

 

[vfone]

Yes, but in 13.56MHz ISM band generally they speak about high power levels, up to 2.4kW, and very short distances.
I've seen a machinery using 13.56MHz electrode RF heating to make plastic bags, and for sure was not QRP

**broken link removed**

 

[kripacharya]

At 100m distance you probably don't need an antenna at all. I can reach most of Europe on 20m band with a short wire antenna.

Brian.

was this CW ?

 

[FvM]

Yes, but in 13.56MHz ISM band generally they speak about high power levels, up to 2.4kW, and very short distances.
I've seen a machinery using 13.56MHz electrode RF heating to make plastic bags, and for sure was not QRP

13.56 Mhz is also used for short range communication and RFID, there are respective ETSI and FCC regulations to assure a halfways clean band.

Field strengths and received power can be easily calculated if you know the antenna properties. Electrical small wire antennas with matching network or magnetical loops can be used. In principle, 100 m shouldn't be a problem, but the 13.56 MHz band usage is restricted to +/- 7 kHz, so any wireless mouse in the vicinity must be expected to use exactly the same frequency and dwart your reach calculations.

In practice, the concept behind short range communication is regulated power and no concurrent transmitters in the "working volume" of the system.

 

[kripacharya]

...
Field strengths and received power can be easily calculated if you know the antenna properties. Electrical small wire antennas with matching network or magnetical loops can be used...

This was my original question actually. How do I model & "match" a small antenna, and how do i work out likely field strengths & received powers ?

 

[Borber]

There are many examples of 13.56 MHz magnetic loop antennas with design instructions.
If bandwidth is in range of few kHz you can use super regenerative receiver.

 

[FvM]

This was my original question actually. How do I model & "match" a small antenna, and how do i work out likely field strengths & received powers ?

I'm using 13.56 MHz only in short range (up to a few m) where the EM problem reduces to a purely AC magnetic which can be easily calculated. Besides specific papers, magnetic antennas are discussed detailed in text books like Balanis Antenna Theory. (See Balanis 3rd edition, chapter 5 Loop Antennas).

Also the parameters of small electrical antennas can be derived from basic text book formulas. And of course, there's much literature from the radio ham side.

 

[kripacharya]

Thanks for all the digressive discussions. Very educative.
Here's what some of my "research" has turned up :

a) there is a 'formula' to calculate antenna resistance (I am hoping its radiation resistance) for short whips. Here's one website

Knowing this 'resistance' I can build a simple matching network. L low pass seems to be OK.

b) short antenna also have a net capacitance ( Xc > Xl ). This seems more problematic to manage.

The 'classic' approach appears to be to use a loading coil which provides an Xl = Net Capacitive reactance in series. These will supposedly resonate and cancel.

HOWEVER the problem with this approach is that a lambda/ 10 or lambda/20 whip has a large Xc. Hence the inductance required is large. Hence the Q is very high.

Hence it is likely to fail very quickly !!!

Is there really no other decent/ stable/ practical method available ?

 

[davenn]

I asked these questions earlier to which you didn't answer .....

All this really begs the questions ....
Why does the OP only want to go 100m using the 20M ham band ?
Is he a licenced Amateur operator ?

If he is, it would be much easier to use a 2M or 70cm freq to get a signal across a short distance like that

so Why that freq ?
are you a licenced ham?
2m or 70cm would be much better, as the antennas are physically very small

cheers
Dave

 

[vfone]

Knowing the radiation resistance of the antenna don't tell you too much about antenna complex impedance (R and j) parameters actually you need to design the matching network.
Again, I would not go for a whip antenna for this relative low frequency (14MHz). The series inductance added to the antenna to improve matching will decrease antenna efficiency. Generally whips are good for frequencies greater than 50MHz.

 

[kripacharya]

I asked these questions earlier to which you didn't answer .....



so Why that freq ?
are you a licenced ham?
2m or 70cm would be much better, as the antennas are physically very small

cheers
Dave

I didn't answer because they are off-topic. Thanks for the tips though.

- - - Updated - - -

Knowing the radiation resistance of the antenna don't tell you too much about antenna complex impedance (R and j) parameters actually you need to design the matching network.
Again, I would not go for a whip antenna for this relative low frequency (14MHz). The series inductance added to the antenna to improve matching will decrease antenna efficiency. Generally whips are good for frequencies greater than 50MHz.

I would think that R of your complex impedance consists of Rrad + Rloss. Matching for this R is less of a problem as already stated.

But being a 'short' antenna that the j would be negative (some high-ish number) for lengths which are < 0.1 x lambda. This is the real problem area imho.

 

[FvM]

I think, a series circuit of Rrad and Xc is a suitable equivalent circuit for the short whip. Some skin effect losses should be added, but they are probably neglectible compared to matching circuit losses.

As concluded in post #16, the matching circuit (series L) will have a high Q and respective low bandwidth. And no, there isn't an alternative, better matching circuit. You just summarized what can be read in antenna theory text books. That's the price of an electrical small antenna. It can be suitable for your application though.

In early years of RC model flight, 13.56 and 27.12 MHz has been used with small whip antennas.