Is velocity factor independent of frequency in coaxial cable?

 

That's a question I asked on my local club FB page, here are the answers. Anyone know better?

http://www.radio-electronics.com/.../coax_velocity_factor... "Hope this helps".
 Coax cable velocity factor is the speed an electromagnetic wave travels along a coax cable relative to the speed in a vacuum.
"I don’t think so but if the frequency is outside the capabilities of the dielectric, then it might have an effect. If the frequency is within the capabilities of the coax, I would say no".
"Not sure Dave, it is dependant on the dielectric used, however I have never seen cable where they quote velocity at a specific frequency".
Good Question, http://owenduffy.net/transmissionline/concept/mvf/index.htm Measuring the velocity factor of coaxial transmission line Implementation of some transmission line systems… owenduffy.net
"When we used to set up phasing harnesses we used to use a vector analyser to get the right lengths but we where being super critical. Thus did not rely on manufactures specs".
"There’s nothing for frequency in the velocity equation but it might be worth finding out if it’s used in the dielectric constant equation. Thinking about it, a good dielectric should look like the correct impedance within the working frequencies".
"I've cut two lengths of coax for use in a 144Mhz Wilkinson divider. One slightly longer than the other. The longer one is a 1/4w*vf and dips at 138MHz and the other shorter one which is vf less 10% dips at 144MHz. Just need to confirm this is right, don't have a vna, access to a antenna analyzer might help. Only reference to this I can find http://www.dg7ybn.de/Phasing/Phasing.htm DG7YBN - Phasing Stacking and matching with coax lines by DG7YBN dg7ybn.de".
"Are the legs made of the same coax?"
"Same coax RG179. VF is measured at 1MHz apparently and it seems to be assumed that it is fixed figure".
"I would think that for all practical purposes, you should be able to assume that. And besides, 144MHz is way higher than 1MHz and not much different to 138MHz so it probably doesn’t make enough difference to worry about".
"Trouble is I'm combining two 300watt amps and would like to be sure it's right but perhaps its not that critical, I'll find out⚡"
"Found an article for adjusting vhf phase in lines using a t adaptor 50 ohm load and swr meter will bring it up".
"Using the club analyzer I've reached the conclusion that the VF of the coax I'm using is 65% and not 69.5% in the data sheet. This ties in with the dip meter and swr tests although the results are not as clear as the MFJ. If the VF is indeed 69.5% at 1MHz which is where it's measured apparently, and 65% at 144MHz it cannot be independent of frequency me thinks". 👍
"Or is the test equipment varying with frequency? I would be highly surprised to find an swr meter flat, mine are not I know that much. As for MFJ, I bought the expensive one it is (from a professional perspective) junk".
"You have to make the best you can from what you have. I'm not a professional but an amateur. This is the swr test".
"I know. I long for a suite of professional grade test kit. I am merely cautioning about not believing everything your test kit tells you as accurate".
"Understood, that's why I've used various test methods. All indicate that the VF is less than the data sheet states so I had to decide on a final figure".

"Here's a photo of the mfj connected to a final cut of coax. Impedance is low but swr high but that's how it should be so I'm happy with that ".☺
 THANKS TO THE ORIGINAL AUTHORS FOR THEIR COMMENTS

Scrapbook photos

Some photos uploaded from my mobile phone and stuck here because it's a scrapbook.

The Shack

Some QSL cards for Two Metre contacts

The old locator system and some squares worked in the early 80s

My elevation indicator!

Six & Four transceiver

An EME contact

New Driven Element

I've changed the driven element of my DK7ZB to the DG7YBN (K6STI) style. This has the advantage of a 50 ohm direct feed eleminating the need for the 50-28 Ohm DK7ZB match. I've had problems water proofing the old match and not been able to source decent 75 Ohm coax anyway.
VSWR is very low and as far as I'm able to tell there is no loss of gain. Proof of the pudding will be in the eating.  EME contest this weekend so if I can make a few contacts I'll be happy if not I might have to reinstall the old DE.

The DG7YBN driven element

Update 15th October 2017

First EME contact with the new driven element

And it seems to work

---

Revieved this nice comment from Hartmut 

 On 01/11/2017 23:07, Blogger Contact Form wrote:

Nice modification on the 2 m Yagi there!
A number of low impedance Yagi are suitable of having a bent driven
element replacing mathcing devices like a T-Match or parallel coaxes.
And this also works out in the NEC model.

Well done.
73 de DG7YBN



Here's a link to Hartmut's website
DG7YBN - Low Noise Yagis

FT8 on Six Metres

Started the Sporadic E season this year using JT65A again, never had any idea that FT8 would be coming along.  Just happened across it somehow, really can't remember.  Downloaded WSJT-8 and joined in the fun.
FT8 really comes into it's own on Six where short lived openings would probably be missed with JT65.  FT8 apparently looses some sensitivity compared to JT65 but the great advantage of quick fire contacts more than makes up for that.  Auto sequencing helps to speed things along. Wasn't too sure about that at first but soon subscribed as the fifteen second over can leave you floundering unless you are very quick and I wasn't.
If I'm around the shack but not able to operate I often just leave things running to see what is about sometimes with surprising results mostly capturing random calls form the continent when otherwise the band is dead.

PET HATE: stations sending RR73

Here's a nicely flowing FT8 QSO on 50MHz

Nicely flowing FT8 QSO

For FT8 I'm using a homebrew Six & Four Metre transciever, HB9CV antenna and 100 Watts.

RF interference noise from garden solar lights

Below is a video of the RF noise generated by three strings of 20 LED solar garden lights located in a neighbours garden. Cheap Chinese imports. The noise is S9 on Two Metres and my antenna is 200 feet away.  The interference is strong enough to be heard no matter where I point my beam including at the Moon.  Of course there is no noise during daylight, one small mercy I suppose!  70MHz & 50MHz are also affected, not sure about HF as I'm not really active there but I'll check soon.

Search using "Simple solar circuits" should give results that may help explain why they generate so much interference.

Here is a screen shot of 50MHz, the peaks of noise are sharper than on 144MHz and drift downward in frequency.

The RF noise drifting across Six Metres

I've purchased some clamp on ferrite suppressors. I will speak to my neighbours and hope they will be willing to fit them but I'm not confident that even if they are fitted the noise will be suppressed to the extent that it's not detectable. Even very low level noise will be a problem with EME.

UPDATE; The noise is also on HF 

Small peaks of noise are detectable on HF but appear fairly static and so if outside of a band they may not a problem. I'll update with a screenshot soon.

WSPR spots while using dummy load

QRP-LABS U3 1Watt into dummy load

Having destroyed the BS170 FET in my QRP LABS U3 I decided to press a small driver amp I'd made into service instead of replacing it.  Wasn't sure what power out I would get and was pleasantly surprised with the easy 1 Watt it produced driven directly by the  Si5351A synthesiser.  The driver was running in class A so no easy way to turn the power out down but nevertheless I decided to soak test and left the U3 running into a dummy load for several hours.  Really don't know why I decided to check WSPRnet for spots and was amazed when several came back.  As you can see in the photo nothing was screened. Perhaps there is a warning there for the next time I start whistling into a dummy load

The spots I received

Pskreporter and DX

Don't you just love PSKreporter, tells you all the DX you've missed while your out and about. Here's what my radio received today on Six. Best of the bunch A6VQ TY2AC AB1NJ. Oh well, most likely wouldn't have worked them anyway.