G5RV Jr. on the Antenna Analyzer

[bandarr3000]

I just got my MFJ-259C in the mail today and I'm wondering if it's broken or if the G5RV Jr. is just a weird antenna.  Maybe both?  Anyway, I'm getting readings I wouldn't expect.  In the 40m and 20m bands I'm getting SWR > 3, and yet my FTDX3000 can tune the G5RV Jr. on these bands.  What's more, where the SWR is the lowest (around 15.1 MHz) I get the most coax loss (>12db), and in the 20m band, for example, the coax loss is extremely low (< 2db).  What gives?  I'll look into the math, but I'm just wondering if things are completely out of whack, or, since the G5RV-type antennas are a compromise, I'm seeing expected results.  Thanks. 

[Josh]

Hi, do you have ferrets on the coax? If not, place some at the ends to damp waves off the outside of the coax, this will also reduce noise flowing from inside the house and near field to the antenna and then back down to the rx. The loss in the coax stem from the length of the coax and z of the antenna, wich of course will be frequency dependent. I used the tuners in my hf rigs to feed my off center fed dipole and some would even tune to 160, depends on the rig and the x it sees from coax and antenna.

Here's a good look at the g5rv;
https://www.w8ji.com/g5rv_facts.htm

[Exo]

Hi, do you have ferrets on the coax? If not, place some at the ends to damp waves off the outside of the coax, this will also reduce noise flowing from inside the house and near field to the antenna and then back down to the rx. 

Ferrets are great pets, they improve your antenna performance.

[Josh]

Here's empirical proof ferrets work wonders in the field of radio;
https://www.youtube.com/watch?v=iZeJgI0mi5o

[bandarr3000]

I do have ferrites on the coax (which is a mix of RG-213 and LMR-400).  I have also run 30 feet of the coax (RG-213) underground (about 6-8 inches deep).  Also, I have a lightning arrestor near the middle of the coax grounded to a ground rod that runs 8 feet deep. 

[bandarr3000]

I should add that I made contact with W6R on 40m last Sunday, and I was told that my signal was 5-9.  So, I'm getting out.  I'm in Grand Rapids, Michigan and W6R is in Springfield, Missouri.

[Josh]

Something about tuners mystifies people. Someone I know, who should know better as a trained electronics and radio tech, said a tuner simply fools the radio into thinking it sees 50 ohms.

This is not the case.

A tuner, or transmatch, is often a two port device that transforms the z seen by at least one of the ports. Also, the power that would be reflected by a high swr as seen by the transmitter end is re reflected back to the load until it leaves the load because the transmatch becomes the proper termination seen by each port - the low z side sees low z, the high z sees high z and the transmatch handles the transfer of energy between the two. And it's a two way affair, optimising power transfer between the ports regardless of source.

Something else to keep in mind is only resistive losses can consume power, caps and inductors - as found inside most transmatches - can't. Sure you can mistune a transmatch and make the antenna system perform poorly, but that doesn't change the fact that with a properly tuned transmatch, little if any power is wasted from one side of a port to the other.

Most hf rigs have a simple T net as a tuner topology, low parts count and high range of swr, more or less becomes a high pass filter when tuned. A pie transmatch would be a bandpass filter as I recall. The newer auto tuners use banks of caps and inductors selected by relays, and the brains will select from them all in a variety of ways to get the most efficient match - these are often L circuits and will swap between cap and inductor depending upon the frequency range. Regardless, you want the least inductance cranked in, most output cap as possible, and whatever input cap is needed for the lowest swr, if you run a manual T tuner.

Sadly, the inductance ranges are typically hard coded into any given hf rig so if there's not enough or too much for a given band, the rig gives up on autotuning. It'd be nice to see the makers roll out bandpass transmatches that have much greater ranges in new hf rigs.

If you've not seen this vid on swr, it's kinda cool;
https://www.youtube.com/watch?v=DovunOxlY1k

[KaySeeks]

I just got my MFJ-259C in the mail today and I'm wondering if it's broken or if the G5RV Jr. is just a weird antenna.  Maybe both?  Anyway, I'm getting readings I wouldn't expect.

Josh already put up a link with an SWR profile. There are others for the G5RV out there; just do a search for G5RV or its derivatives (e.g., ZS6BKW). It is nowhere close to "flat" from 3-30 MHz.

Aside from the variations in VSWR that this antenna presents, since you are questioning your analyzer readings, you didn't mention so I have to ask, did you connect to a known Z or R and get a reading sufficiently close to expectations? When readings don't make sense, always, always, always go back to a known reference, e.g., a dummy load.

[KaySeeks]

Ferrets are great pets, they improve your antenna performance.

Ferrets are used for other things, but this is the family programming channel so we won't get into that here.

[KaySeeks]

Someone I know, who should know better as a trained electronics and radio tech, said a tuner simply fools the radio into thinking it sees 50 ohms.

This is not the case.

A tuner, or transmatch, is often a two port device that transforms the z seen by at least one of the ports. Also, the power that would be reflected by a high swr as seen by the transmitter end is re reflected back to the load until it leaves the load because the transmatch becomes the proper termination seen by each port - the low z side sees low z, the high z sees high z and the transmatch handles the transfer of energy between the two. And it's a two way affair, optimising power transfer between the ports regardless of source.

Optimal transfer to/from the radio is when it is presented with ~50+j0 Ohms. The antenna tuner/transmatch does the best it can to approach that, when tuned properly. In other words, it fools the radio into into thinking it is seeing as close to 50+j0 Ohms as possible.

Also, optimal transfer to/from the antenna likely happens at some impedance other than 50+j0. The tuner/transmatch does the same "fooling" bit for that side too.  Imagine that. 

On a related subject, I don't understand this complete and all-out attempt to avoid the usage of a transmatch/antenna tuner in ham radio. The way I read some articles, I come away with the feeling that some authors feels like it's utter failure to have to use a tuner, which is ridiculous to me. I get not wanting to have to lug one around if you are backpacking or something but otherwise face up to the physics and suck it up, buttercup. You need to match to the load or accept the losses.

[bandarr3000]

I totally forgot about the dummy load.  Hooked it up to the analyzer and got a 1:1 SWR.  The G5RV Jr. is not resonant on any ham band.  But as long as it tunes, I'll keep it up for a while and make notes.  I've got a AlphaDelta DX-EE waiting in the wings. :-)

And thanks for the video.  That was the best explanation I've ever seen, heard or read on SWR.

[refmo]

Some food for thought when dealing with lossy transmission lines (and lossy antennas)....
Transmission lines with high losses will give false readings of low VSWR due to the losses attenuating the signal before it gets back to the meter.
If your 12db loss is one-way, then the round-trip loss for the reflected energy will actually be 24db. So even if the VSWR is actually infinite (all the energy gets reflected), the reflected energy level will be 24db (about 15 times) less than the original signal by the time it gets back to your meter (which will give you a very good reading).  Even total losses of 12db would give a fairly low SWR reading.  The more losses you have in  the cable, the better your SWR will 'appear' to be with a meter located at the source.

Lossy transmitting antennas will cause similar problems (energy is dissipated in the antenna (unwanted) instead of being reflected (usually also unwanted) or radiated (desired effect)).
Just because the SWR of a transmitting antenna appears to be good does not mean it is a good radiator.
(Some receiving antennas have losses which can give desired effects such as directionality/improved signal-to-noise).

[KaySeeks]

Transmission lines with high losses will give false readings of low VSWR due to the losses attenuating the signal before it gets back to the meter.
 ... The more losses you have in  the cable, the better your SWR will 'appear' to be with a meter located at the source.

Yes, completely true.

To the O.P., You may want to connect your antenna analyzer to the antenna at the feedpoint (if possible) and avoid the coax to get a true reading of the antenna. Assuming it doesn't involve gymnastics, I prefer to do that instead of "backing out" (calculating what the antenna really looks like) through software or a Smith Chart and avoid possible human error.

Then, put a dummy load at the end of your coax and connect your analyzer to the other end of the coax. You should see loss but hopefully you will see something like Z = 50+j0. This is a good check to be sure that your coax isn't screwing things up more than it should. Find out what the expected loss (per unit of distance) is for your coax and see if the loss makes sense.

[Josh]

Yes, check the ant at the feedpoint for swrs, as well as at the far end of the coax. This will reveal the truth of the matter as seen by each node of the antenna system, a transmatch is a systemic device, it tunes the overall antenna system; coax and antenna together if placed at the output of the transmitter. And now for a word from my (sk) elmer, Walt;
http://www.k6mhe.com/files/Reflect_rev1.pdf

[bandarr3000]

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.

Then, I connected the analyzer to the G5RV Jr. via a 3 foot patch cable. That's when things got interesting.  I got the following SWR readings:

Freq - SWR
6.925 - 3.9:1
7.000 - 3.8:1
7.300 - 5:1
10.100 - 20.1:1
14.200 - 5.4 : 1
15.540 - 1.1 : 1
18.100 - 7.6 : 1
21.300 - 22.1 : 1
24.900 - > 25:1
28.500 - 1.7:1

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!