Measuring with 2 types of cable in line ?

[ve2fu]

HI I am waiting for my unit to arrive ! in the mean time I have been reading every possible info... now I have this question.

How can I see the antenna at the other end without going up the tower.
As most of us do ... I have 2 type of coax, a run of 80 feet heliax going up the tower and a 15 feet of RG-213 (for the jumper and 9 turn x 6 inch DIAM coaxial balun) connecting to the antenna terminals.

Is there a way to enter this data to the AIM4170 for measuring the antenna.

My second question: I have a directional coupler made by NARDA. Can I have a set up so I can make measurements like the two port vna does. (meaning BAND PASS FILTER sweep). What would be needed for a simple set up ?

Thank you PHIL VE2FU

[Bob]

Hi Phil,

It is possible to enter data for a single piece of coax but complex systems, especially with baluns or filters, need to use the "custom calibration" procedure. This takes into account any number of sections of transmission lines and filters between the AIM and the measurement point.

This does require putting loads at the feedpoint of the antenna. Once it's done the calibration will be stable and you can change the scan limits without calibrating again.

The AIM can be used to get an idea of filter performance. It's not as good as a two-port VNA but for a quick check, it's ok. I'm beta testing this function now. It shows the rejection down to about -40dB and it's useful on filters that are fixed, ie, not tunable. The procedure is not fast enough for making filter adjustments in real-time. Anyone who's interested in trying it, please contact me by email and I'll send info on the procedure.

-- Bob

[ve2fu]

ok

Thanks Bob I was expecting this answer so I will do a calibration at the far end of the line.

Now can I save the calibration made on different line for future use ?

For the BPF performance yes I am interested in such evaluation and will send you an e-mail for that purpose but, I thank you for answering this question as well and wait till I have played with the unit.

Tnx again 73' Phil VE2FU

[Bob]

Hi Phil,

You can save as many cal files as you like. You can use one custom cal file to cover all the HF ham bands if you want to and then change the limits to zoom in on each band later. It takes a little longer to do the custom cal, but when you're scanning a band, it runs just as fast, whether you're using custom cal or the standard cal procedure.

--73/ Bob

[w0qe]

Phil,

The answer depends on your required accuracy. With your configuration there are several variables that would need to be known to get an accurate answer. The length of the each piece of coax, the loss, the actual impedance of the coax, and the velocity factor. Also connectors and their placement such as UHF types (which look like 0.9" of 30 ohm transmission line) would also need to be known.

Of all these variables the largest unknown IMHO is the velocity factor (VF) which varies quite a bit compared to the impedance and loss. Of course loss does increase a little with age. I have measured variations in VF for many types of teflon coax from .685 to slightly over .700. While this may not sound like much it changes the electrical length of the coax by over 3 feet in a hundred foot length which is why folks making phasing sections or stubs want to use coax from the same roll. A 3' error in length is about 30 degrees of electrical delay at 10m which is very significant.

My advice would be to climb the tower. Remember that you will need to have an open, short, and known impedance to do the calibration. If your coax terminates in pigtails that are a few inches long them get a 1/4w 1% resistor somewhere between 50 and 200 ohms. Cal. including the pigtail which will give you the most accuracy. Also the custom cal. is more accurate than the standard cal. and do it for the entire HF range in case you connect something else later to the coax. A suggestion would be to do a custom cal. from 1 to 60 MHz in 250KHz. steps. When done make sure you save the calibration for future use.

73, Larry W0QE

[ve2fu]

Great !
Thank you for your help Bob and Larry, I will follow your advise and do those custom calibration on every line. Just got confirmation of delivery ! so let you know in a week how it went.
73' Phil VE2FU