HF Tx Line Testing with NanoVNA


In previous posts I tested the various HF/VHF/Microwave antennas that I have using the versatile NanoVNA (Ref.1/5). For the past six months my apartment balcony has been under construction and finally it’s finished. Unfortunately when I setup my HF Buddipole antenna there was no reception, so I suspected that the transmission line which was left on the balcony during this time had some problems. So I decided to test the line and sweep it with the NanoVNA. A quick short circuit test measured 20 ohms which was obviously a problem.

Test Procedure

Fig.1 Tx Line Testing Diagram

Figure 1 shows the test setup to check the cable. Using a digital volt meter at the receiver end of the cable, I measured the DC open circuit and short circuit resistance of the cable. When I first shorted the cable at the antenna end, I measured 20ohms. The RG-58 cable has a centre conductor resistance for AWG20 of about 10ohms/1000ft, so for 40ft this should amount to about 0.8ohms (2x.01x40ohms). Clearly there was a problem. I then used an emery board to sand the centre pin of the PL259 and the collar (they were caked with construction dust).

Test Results

Resistance of DC Short at Antenna 0.8 ohms
Resistance of DC Open Circuit at Antenna>20Mohm
SWR 100KHz – 30MHz<1.25
Fig.2 Tx Line Tests
Fig.3 Tx Line VSWR

After sanding off the PL259, the DC short was reasonable and the DC open was infinite so that was good. I then placed a 50ohm termination at the antenna end of the cable and swept with the NanoVNA. Figure 3 shows the SWR which was <1.25 from 100KHz to 30MHz. My 20 year old balcony cable consists of 3xPL259 + 1xSO239Thru + 1BNC so not exactly precision 50ohms, but adequate for receive only for now.

Fig.4 HF Transmission Line Testing with NanoVNA

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By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.