4170 and really long coax?

[denniskb7st]

Hi,

I think I got a really good deal on eBay. 700 feet of West Penn .82VF RG-59 was shipped to me by mistake. I ordered their 700' of RG58 but they sent me RG-59 instead. Now they will let me keep the RG-59 and ship me the 700 feet of the RG-58 at no extra cost. I'm putting up a 40M sloper and will try this RG-59 first. Run will be around 150 feet to the tuner/rig.

I just swept the 700-foot spool. DTF reports 469 feet, Zo of 78 ohms. Used both STD CAL and my Custom Cal with BNC-banana jack adaper. Differences don't look significant.

I assume the 4170C can properly measure lengths greater than the 470 feet it measured on this 700-foot spool, right? So, that means the cable is defective somewhere near the 470 feet, right? If it truly means a fault, I guess I shouldn't complain because it turned out to be free. I'm pretty sure the coax has the length printed on it every foot so we'll see what happens when I examine 470 feet from where the AIM tested it.

Dennis KB7ST

[k6mhe]

What is the Velocity Factor for your coax? If its around .67 VF (700 * .67= 469) your measurement seems correct.

Danny, K6MHE

[Bob]

Hi Dennis,

The Distance to Fault function in some earlier versions of the AIM program did not handle long cables properly. Make sure you have the latest version.

Do a custom cal on your AIM from 50KHz to 2MHz with steps every 10KHz. Then scan the cable in the usual way from 50KHz to 2MHz. You should see some phase zero crossings and these frequencies will be listed at the top of the graph as "resonant freq".
The first phase zero crossing, indicating the quarter wave freq, should be around 200-300 KHz.

Use the first freq to calculate the distance to the quarter wave point:

Length of cable (feet)= 246*Velocity_factor/Freq(MHz)

Length in feet; first resonant freq in MHz

For example:

First freq = 0.232 MHz,
Velocity factor approx 0.66 (this can vary a lot)

Length = 246*0.66/0.232 = 700 feet

If the resonant freq is higher than this, the cable is shorter than 700 feet
(or the VF is different).

You can also check the VF by cutting off a short piece, say 10 feet long, and measuring it by itself.

-73/Bob

[denniskb7st]

Hi Bob,
Scratching my head a bit at these results attached.

I did the recommended custom cal, Bob, using the AIM the three supplied AIM cal loads. I also cut a 10-foot length and did an iterative changing of VF of plus/minus .82 (as published by the manufacturer). Using .833 provided the closest result of what physical length I cut.

Stripped coax and did not attach PL259. Instead I used my bnc-banana adapter (previous custom cal). With this adapter, got good agreement on VF of .833 and actual physical length.

Then grabbed the spool of RG-59. After posting, I used about 200 feed to feed a 40M sloper. So, if the seller is correct about selling me 700 feet, there should be about 500' left on the spool. I stripped the cable a bit and then clipped the center and shield to the correct parts of the bnc-banana adapter.

Then I did the normal scan and file attached. First theta crossing appears to be about .0833 mHz. Way below the expected. As you can see, the scan looks kinda noisy. I have a 25KW AM BC station about 6 miles to the west and wonder if that has made the plot dirty.

When I used the formulas with the .0833, well you know the results are nowhere in line with the 500 feet expected on the spool. I'm not sure how to resolve the issues now.

Can't see how to attach the AIM scan file in this forum message.

When I then did DTF on the sppol , I got a message Cable seems lossy or it is terminated in a load. maximum theta 34.7 degrees.
Dennis
PS--I got the AIM last spring and finally got back on HF within the last month or so after getting a barebones Elecraft K3. So, AIM is getting much more usage.

[Bob]

Hi Dennis,

If the BC station is a problem, it will probably depend on the orientation of the roll of coax. Try moving it around and see what the result is.

Is it practical to check the 200 foot section you're using? Can you disconnect the center wire from the antenna or short across the antenna feed point for a scan test?

Check your AIM by scanning a resistor of a few hundred ohms over the range of 0.05 to 1MHz. It should result in a flat line for Zmag and Rs.

You can send your pictures to my email address.

-- 73/Bob

[denniskb7st]

Bob,
I'm OK now. I took your advice and did a custom cal on my BNC-banana jack adapter using a 385-ohm resistor in 10k steps from 50k to 2 megs. Then scanned it and it looks wonderful. Also then did 98-ohm resistor and it looks good. Now the cable shows first theta crossing and reported resonant frequency of 810 kHz. DTF now shows fault at 253 feet, but I've taken about 200 feet off for the antenna and a little more for the 10-foot cable testing. With the 10-foot cable, determined VF was closer to .833 than manufacturer reported .82. With our new rain storm near Seattle, inconvenient for me to lower the 40M sloper and test the cable after shorting it there. I thot I cut the dipole pretty spot on for length and see AIM scans showing theta crossings 6.05 mHz and also 7.5.

Different issue ~ When we look at theta 0 crossings on either side of an antenna, how do we decide if the antenna is too long or too short? Only key difference I see between the two theta 0 crossings is that Zmag is lowest (24 ohms) at 7.5. Zmag is 225 at the 6.05 theta 0 crossing. a 40M modeled sloper is supposed to be around 100 ohms impedance, I think.
Dennis

[denniskb7st]

Here's the rest of the story ~

I took the coax of the spool and it turns out that the spool only contained about 430 feet instead of 700 feet. I had figured they shipped me 700 feet but that there was some defect where AIM's Distance To a Fault predicted. But no, there was never anything close to 700 feet on it. Now, I'm waiting for the replacement spool of the RG-58 and the first thing I'll do is turn AIM on it.

Dennis