How can I measure an antenna trap and other questions.

[HoracioDos]

Hello.
I want to build a multiband dipole antenna (40/20/15 meters) for a SDR dongle + upconverter (Rx Only) using only one wire for each side.
I found in the web how to build the traps for each band using rg59 or similar coax cable. There is also an small sofware available that allows to calculate the coil lenght  based on coax specs, pvc pipe diameter, etc.
So my question is how can I measure if the coils are correct. I don't have a VNA but I can borrow a LCR meter. Is it possible to do that with a LCR meter?
I have a related question about the balun. I read that it is possible to build it (1:1, 4:1, 9:1) using a toroid from an old pc power supply. How can I know if it is the correct one. (I gues it could be the right one in the picture)
Last question: Do you know a good basic antenna theory book for non tech?.
Thanks in advance.

[tkamiya]

Please keep in mind, an antenna cut for 40 meters (dipoles, etc) will resonate for to meters as well.  For 20 meters, you could add another pair of elements like fans.  Traps are harder to make and strictly speaking, lossy.

To make traps, you'll have to achieve resonance of the coil/capacitor pair to be the higher of the frequency.  Resonance will be very critical, so you can't strictly go by calculated numbers.  You'll have to build the circuit and use dip meter and tune it in.  Coax based traps are actually harder to make than usual coil kind.  If you insist in using traps, my recommendation will be a coil and a trimmer/variable capacitor.  Tuning will be a lot easier. 

If this is going to be for receive only, I'm going to suggest not bothering with traps.

For antenna making, ARRL antenna handbook is a good book to have.  Plenty of theory, plenty of examples, and plenty of tricks hams used over time.  (yes, it is actually a BOOK!)  I know it comes with DVD but I don't think there is an online version.

[cdev]

Consider that when using a trap, you're actually not using some or most of the antenna. A unun or balun (that performs an impedance transformation) and/or some kind of matchbox often makes much more sense. Since you're using it for receiving you can use tiny, cheap parts. This is what I have done. If you just want to use a single wire antenna, plus counterpoise, I'd start with a 9:1 unun.

You need three pieces of insulated wire from a Cat 5e or 6 Ethernet cable (any wire will do but they are a good choice for low loss and the impedance is about right.

 If you have two identical ferrite beads (which are usually #43) that will work best.  Use the typical 9:1 unun winding except since you'll be using the equivalent of a binocular core you need to wind fewer turns, and they should go, together, through both cores. Experiment to find the optimal number of turns. Its really remarkable how much that improves reception.

That's your starting point. At the receiver end you can build a tuner / preselector. If you have a big problem with AM broadcast noise use a matching design that acts to roll off lower frequencies. The opposite if your problem is with higher frequencies.

[bob91343]

Why not spend the few dollars to buy a nanoVNA?  They could easily make your desired measurement and provide many other useful functions.

[Co6aka]

I want to build a multiband dipole antenna (40/20/15 meters) for a SDR dongle + upconverter (Rx Only) using only one wire for each side.

Before going down that road, take a look at these:

https://www.dxer.ca/index.php/library/category/19-lf-free?download=317:lf-2020-a
https://www.okdxf.eu/lankford/The%20Best%20Small%20Antennas%20For%20MW,%20LW,%20And%20SW%20rev%202.pdf

And more here:

https://www.dxer.ca/index.php/library/category/19-lf-free

[bob91343]

Borrow a grid dip meter.

[Yansi]

I have a related question about the balun. I read that it is possible to build it (1:1, 4:1, 9:1) using a toroid from an old pc power supply. How can I know if it is the correct one. (I gues it could be the right one in the picture)
Last question: Do you know a good basic antenna theory book for non tech?.
Thanks in advance.

Yes, it may be possible, but for middle wave bands at best. The typical cores you can get outta old PC power supply is iron powder mix #26 (yellow-white), which is the cheapest shit ever (permeability about 75) or sometimes mix #52 (green-blue), which has slightly less losses and little less permeability too. Anyway, both can operate likely at best at a few MHz region.
Loss depends a lot, on what the flux swing is. So it may be possible to use at higher frequncy with less flux swing, but that can severely limit as to what inductor/transformer you can make out of it.

#26 and #52 are certainly not RF material.