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Author Topic: G5RV Jr. on the Antenna Analyzer  (Read 1335 times)

Offline KaySeeks

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #15 on: September 22, 2018, 0221 UTC »
I connected the feed line to a dummy load at the antenna end and connected the analyzer inside the shack.  For the most part, I got 1:1, with an occasional 1.1:1. So, for 85 feet of coax I'm pretty happy with that.

The longer the coax run is, the more loss there will be and the lower the SWR will appear due to the losses partially "masking" the mismatch reflections, as others have stated.

In any case, seems like you're in good shape.

Does the analyzer give you any estimate of the loss (in dB) of the cable? If not, don't worry about it.


Then, I connected the analyzer to the G5RV Jr. via a 3 foot patch cable.

The "resonant" frequency appears to be 15.540, with somewhere in the 10 meter band being the next best thing. Can't wait for 10 meters to open up full time!

This is along the lines of some of the articles on G5RV antennas that I have read - best resonance somewhere 11-16 MHz, depending upon the situation.
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Offline bandarr3000

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #16 on: September 22, 2018, 0237 UTC »
Does the analyzer give you any estimate of the loss (in dB) of the cable? If not, don't worry about it.

At the "resonant" frequency, with the antenna attached (my only option right now since it's dark outside), my coax loss is 10db. 

I should add that I have two right angle connectors in the mix, plus a barrel connector, a lightning arrestor and two wall plate bulkheads.  Most of the coax is brand new RG-213 with about a 20 foot stretch of LMR400 that has been outside for a couple of years.  I don't expect a significant amount of loss from the RG-213 (even though 30 feet of it is buried) and LMR-400, but I suspect the connectors may not be my friend in this case.

I should also add my coax loss at 14.200 MHz, with an SWR of 4:1, is 2.1 db with Z of 142.
« Last Edit: September 22, 2018, 0250 UTC by bandarr3000 »
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Offline bandarr3000

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #17 on: September 22, 2018, 1529 UTC »
Ran some coax loss tests with the line terminated at various sections with the dummy load and unterminated. 

Unterminated, the entire segment (about 85 feet), connectors and all, produced a loss of about .7db. 

With the dummy load hooked up,  losses were high (about 12 db for the entire segment).  This is where I reveal my ignorance of dummy loads and loss.  When connected to shorter segments, the loss was higher. 
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Online Josh

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #18 on: September 22, 2018, 1926 UTC »
If you have a hf low pass filter (ie cuts off above 10m) handy, place it inline and check swrs again on the antenna system. I find it moderates the radical swr readings from band to band in the case of my obsessive compulsive I mean off center fed dipole.
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Offline KaySeeks

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #19 on: September 22, 2018, 2020 UTC »
If you have a hf low pass filter (ie cuts off above 10m) handy, place it inline and check swrs again on the antenna system. I find it moderates the radical swr readings from band to band

Any filter will have some loss within its passband, while still presenting a "reasonable" (~close to Zo) impedance on either port. The LPF does what you say by introducing loss within the passband inline with the coax. That loss will reduce the impedance variation seen at the far end of the coax. Thus it is "fooling" (there's that word again ;D) the radio into thinking it is seeing an impedance closer to Zo.

It's an old trick; use it all the time in the lab to calm harsh impedances and isolate.

Nothing magical about it. You don't even have to use a LPF. Anything with loss will do the same thing. In this way it's not different than extending the coax by the distance to generate the same amount of loss.
« Last Edit: September 22, 2018, 2048 UTC by KaySeeks »
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Offline KaySeeks

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #20 on: September 22, 2018, 2045 UTC »
Ran some coax loss tests with the line terminated at various sections with the dummy load and unterminated. 

Unterminated, the entire segment (about 85 feet), connectors and all, produced a loss of about .7db. 

With the dummy load hooked up,  losses were high (about 12 db for the entire segment).  This is where I reveal my ignorance of dummy loads and loss.  When connected to shorter segments, the loss was higher.

Depends upon how they calculate cable loss in the analyzer (and here I am showing my ignorance :) ) but it sounds like they are just taking the return loss (which is related to VSWR) and calling that the cable loss. Return loss magnitude is higher with a proper load and near zero with a short or an open as the load. It's a shorthanded way to get a result. However, you would have to use either an short or an open at the other end to make this "right".

Take a look in the manual and see if they ask you to put a short or an open at the end of the coax.

Edit: I said to myself, jeez, just go look this up myself.

Page 19 of the MFJ-259C manual
Quote
6. Defective Cable: Erratic SWR readings will also occur if your coax isn't 50 ohms. Kinks,
water ingress, oxidation, corrosion, bad connectors, improper construction, and even
mislabeling by the manufacturer may be the cause. Check SWR with a dummy load installed
at the far end of the cable. If it is elevated or the Impedance (Z) fluctuates very much as you
tune the analyzer's VFO, suspect defective cable.
7. Lossy Cable: Coax may exhibit excessive loss from contamination or may have too much
normal attenuation for use at higher operating frequencies. To measure loss, unhook the cable
and use the analyzer's Coax Loss mode to check it against the factory specifications
.


So I was right. Ignore the loss value with a dummy load attached. Use an open end with "Coax Loss mode".
« Last Edit: September 22, 2018, 2050 UTC by KaySeeks »
Just somebody with a radio, a computer and a pair of headphones...

Offline bandarr3000

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Re: G5RV Jr. on the Antenna Analyzer
« Reply #21 on: September 22, 2018, 2139 UTC »
So I was right. Ignore the loss value with a dummy load attached. Use an open end with "Coax Loss mode".

Excellent.  Thank you for this.  A loss of .7 db over 83 feet is ok by me.
Yaesu FTDX3000D
SDRPlay RSP1A
W6LVP loop antenna
EFHW 40-10 as inverted L
Grand Rapids, MI

eQSL please to bandarr@gmail.com