Reflected power readings

[Riktar]

Both of my "commercial"  premade antenna cables (LMR400 - 90ft and 15ft) show a reflected power reading of 0.23 and 0.05 respectively on my Nissei RS-50 meter. The forward power is 24 watts and the swr is 1.15 between the 2.

 

I tried my hand at crimping a PL-259 connector on another cable to make a "custom" length. And since I am still waiting on the stripping tool for this I went online and used the example(s) of a box knife to prep the cable. What a pain that is. I hope the stripping tool gets here soon!

 

Anyway, after doing this and hooking up the meter I am show a minor deviation on the forward power but the reflected power has jumped to 1.68. My SWR has also increased to 1.56.

 

While connector installation looks proper on the cable I am wondering why the increase in reflected power? Could I have done a poor job on soldering the tip of the connector to finalize the connection to the center conductor? 

[berkinet]

Was the original connector also a PL–259?

[Riktar]

In a manner of speaking. I suppose I should elaborate:

 

Both of the commercially made cables are MPD Digital cables that have PL-259 connectors on both ends. The cable I am modifying is a 100 foot LMR400 cable with male N connectors on both ends. Aside from shortening the cable to 35 feet for a better fit, I wanted to install the PL-259 connector to avoid having to use an adapter on my Kenwood 8180 which utilizes the PL-259 connector for cable attachment. The antenna I wanted to try and use has a female N connector so I thought adding the PL-259 connector on the LMR400 would allow me to avoid having to use adapters to connect the antenna to the radio. 

[Riktar]

Sorry for all the edits to the above post.

[WRAK968]

You likely would have done better using a N female to PL259, MPD digital cables basically have a lifetime guarantee, but if you cut them you lose that

LMR400 also crimps a bit differently. You need to have the proper connectors and tools to make sure the crimp is correct with as little loss as possible. My experience is that your better off with soldier/crimp combinations when it comes to LMR cables. You have to make sure the cable is cut cleanly (Using a pair of shears is not advised) and accurately (1/16" of an inch off with your shielding can cause a lot of issues with reflected power) A lot of people will claim the loss is from the PL259 connection added in, but personally I haven't seen much effect unless you have multiple PL259 connectors in a long run of coax, and even then, I think the length of coax has more to do with loss than the connectors do.

Lastly, its very difficult to get a perfect 1:1 in SWR and 0 reflected power. II tell people that if they are less than 1.3:1 SWR they shouldn't continue tweaking their system as its easy to lose ground just as you have.

[Riktar]

The solder/crimp terminals (and the crimp tool) I have are for LMR400.

 

And, well, nuts on pooching the lifetime warranty. Then again, I mis-measured terribly and 100 feet was way more than I needed so I figured I would cut off the excess and experimaent with the shorter 35 foot long piece.

 

Once I get the stripping/prep cutter tool I will give this another go on the "short" section. Hey, at the end of the day there will be a value (to me anyway) in learning about this process.

 

For now my big concern is the increase in reflected power. Could a reading as high as mine cause damage to the radio?

[Riktar]

Just tried another crimp. Pretinned the end of the conducter before putting the crimp/solder on this time. MUCH better results. 

 

I am comparing my 35 foot "practice" piece to a 90 foot premade MPD Digital LMR400 with PL-259 connectors that is also a bit long for what I need but anyway....

 

Now my reflected power is more in line. The premade piece is showing 0.53 with and SWR reading of 1.45 and my practice piece is showing 0.48 and SWR is at 1.40

 

Maybe the tinning helped? Maybe the 2nd time I had practice? In any event I am going to leave well enough alone since trying to wotk with LMR 400 is REALLY fricking hard without the proper stripping/prep tool. At least for someone with my experience level.

 

Thank you all for the replies and info!

[WRAK968]

The solder/crimp terminals (and the crimp tool) I have are for LMR400.

 

And, well, nuts on pooching the lifetime warranty. Then again, I mis-measured terribly and 100 feet was way more than I needed so I figured I would cut off the excess and experimaent with the shorter 35 foot long piece.

 

Once I get the stripping/prep cutter tool I will give this another go on the "short" section. Hey, at the end of the day there will be a value (to me anyway) in learning about this process.

 

For now my big concern is the increase in reflected power. Could a reading as high as mine cause damage to the radio?

We all have to learn at some point so I hope you understand I wasn't meaning anything negative.

 

Almost all commercial radios can handle up to a 3:1 SWR, though I would only run a radio with that high SWR in short bursts or in low power with a fan to help cool the heat-sinks. Below 2:1 and the radio will be fine though you may see a reduction in range which is why people strive for a 1-1.5:1 ratio.

[WRAK968]

Just tried another crimp. Pretinned the end of the conducter before putting the crimp/solder on this time. MUCH better results. 

 

I am comparing my 35 foot "practice" piece to a 90 foot premade MPD Digital LMR400 with PL-259 connectors that is also a bit long for what I need but anyway....

 

Now my reflected power is more in line. The premade piece is showing 0.53 with and SWR reading of 1.45 and my practice piece is showing 0.48 and SWR is at 1.40

 

Maybe the tinning helped? Maybe the 2nd time I had practice? In any event I am going to leave well enough alone since trying to wotk with LMR 400 is REALLY fricking hard without the proper stripping/prep tool. At least for someone with my experience level.

 

Thank you all for the replies and info!

Glad to hear your experience is helping With the right tools it becomes very easy. I am curious, which tools did you order for stripping and crimping?

[Riktar]

The crimping too (and included PL-259 ends came from Amazon:

 

https://www.amazon.com/gp/product/B01E6804LQ/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1

 

It does a reasonable job:

 

 

The stripping/prep cutter I am still waiting on:

 

https://www.amazon.com/gp/product/B00HDPIG4C/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

 

And while I am waiting (and upon reflecting how "cheap" this stripping/prep too) I am pondering ordering this:

 

https://www.amazon.com/Times-Microwave-Combination-LMR400-Cables/dp/B007JSCV12/ref=sr_1_1?crid=3LMA7BQMYQC2E&dchild=1&keywords=lmr400+stripper&qid=1586031073&sprefix=LMR4%2Caps%2C182&sr=8-1

 

Now if this was the only time I would be doing this than I would not consider this. But after wrestling with the LMR400 last night and considering my brother and my daughter want to get a home base setup similar to what I am putting together. Maybe I would be well served to buy this. 

 

Buy quality and you only cry once sort of thing.

[Lscott]

The cable I am modifying is a 100 foot LMR400 cable with male N connectors on both ends. Aside from shortening the cable to 35 feet for a better fit, I wanted to install the PL-259 connector to avoid having to use an adapter on my Kenwood 8180 which utilizes the PL-259 connector for cable attachment.

I think you have two things going on here.

 

1. The connector you installed, PL-259, is not a constant impedance type like the type "N" which will contribute to the higher SWR reading. The connectors calculate out, based on some dimensions on a few I've looked at, of around 25 to 40 ohms. It also depends on what the dialectic material used happens to be as well. The length, connector, of the miss match section also matters but in this case its rather small. This results in an impedance "bump" and a cause for some reflected power. If the radio uses an SO-259 socket there isn't much you can do about that.

 

2. When you cut the cable length from 100 feet to 35 feet you reduced the losses in the cable. However that applies to not only the forward power but also the reflected power. The formula for calculating SWR based on the power reading is:

 

SWR = (1 + sqrt(Pref / Pfwd)) / (1 - sqrt(Pref / Pfwd))

 

Where:

 

sqrt() - square root function

Pfwd - Forward power

Pref - Reflected power

 

How this works to increase the SWR reading as measured at the radio end of the cable is as follows. With lower forward losses the reflected power from any antenna mismatch will be higher because the power to the antenna has increased. Additionally the reflected power is attenuated less as well. Both work to increase the ratio (Pref / Pfwd) in the above formula. Remember you're measuring the forward power at the radio end and that hasn't changed. Thus the numerator becomes larger while the denominator smaller in the above formula. The final result is the number calculated becomes larger, the SWR.

 

I suspect the cable length change has more to do with the increase in SWR than the connector in this case. So as others have pointed out most radios work OK with an SWR up to 2:1, at least that's what I've seen in the spec's for the ones I looked at, without issues. An SWR around 1.5:1 or so is fine. You won't gain much by trying to lower it.

[Riktar]

Thank you for the replay and the info! I will try and digest that and use it going forward.

 

 While I said I wan't going to try any more PL-259 installs on the LMR400 I decided ti do one more this morning. My reflective power is now down to 0.42 and the SWR is holding at 1.37 so I am going to leave well enough alone.

 

 As to your explanation (and again, thank you) about the impedence mismatch between the N connector and the PL-259: Since my Kenwood does use the SO-239 and the antenna has the N, would it be better to just use an n to 259 adapter/connector? From the information I have gotten from others about minimizing the amount of added items between the antenna and the radio I figured that replacing the N connector with a PL-259 would eliminate the need for an extra connector. Or in the case of having 2 different types of connecters, it doesn't make enough difference adding an adapter in vs changing one adapter for another?

 

 And for future purchases and/or installs: Avoid getting an antenna with an N connector and sticjk with SO239?

[Riktar]

One other question then: Other people have stated the DB404 antenna is the pinnacle of performance for a base setup. I have not seen and listing for one with a SO-239 connector. How would a person handle the difference between the 2 types of connector in that scenario?

[Jones]

The mismatch is not between the connectors. Always use type "N" connectors when possible, but if your antenna is type "N" and the radio is SO-239, then using a cable with type "N" on the antenna side, and a PL-259 on the radio side is the way to go. Try to never use adapters in any kind of permanent installation.

 

The impedance bump is on the PL-259, not on the "N" connector, which is an impedance balanced connector.  {By the way, there are 2 different types of "N" connectors out there, depending on whether you are using 50 Ohm or 75 Ohm transmission line (cable).}

 

To clarify your previous question, "...if the radio has an SO-239, should you also get an antenna with an SO-239 so they match?" ...the answer is NO. Always use "N" connectors for UHF when available, but if SO/PL is what the device has, use it.  Having different connectors on each end of the feedline is not the problem, and having a slight mismatch on one end is better than having a slight mismatch on both ends.

[Riktar]

Thanks for the info!

 

 This forum (and the members of it) are great!!

[Solly]

The Microwave Times is what I use at work every day. The cheaper one will work also, but I was more prone to mistakes with it. Always test the conductivity and be sure there is no cross conductivity, prior to installing your coax.   

[Riktar]

The Microwave Times is what I use at work every day. The cheaper one will work also, but I was more prone to mistakes with it. Always test the conductivity and be sure there is no cross conductivity, prior to installing your coax.   

Oh you betcha! I have my VOM ready after every crimp/solder/twist.etc. Standing by to verify a good connection!

[n4gix]

If you think that $99 tool is expensive, check out the price for a stripping/prep tool for 1/2" heliax. I got 640' of Andrews FSJ4-50B for free a few months ago. It's just sitting my garage waiting for nicer weather...

CommScope Automated Tool for FSJ4-50B - $153.42 (includes shipping)
https://www.amazon.com/CommScope-Automated-Tool-for-FSJ4-50B/dp/B007JSCJ1E/ref=sr_1_22?dchild=1&keywords=stripping+tool+1%2F2%22+heliax&qid=1586803654&sr=8-22

 

EDIT: I just ordered it since I just noticed that I had a $25 worth of rewards points on my Amazon Visa card...

[Riktar]

If you think that $99 tool is expensive, check out the price for a stripping/prep tool for 1/2" heliax. I got 640' of Andrews FSJ4-50B for free a few months ago. It's just sitting my garage waiting for nicer weather...

 

CommScope Automated Tool for FSJ4-50B - $153.42 (includes shipping)

https://www.amazon.com/CommScope-Automated-Tool-for-FSJ4-50B/dp/B007JSCJ1E/ref=sr_1_22?dchild=1&keywords=stripping+tool+1%2F2%22+heliax&qid=1586803654&sr=8-22

 

EDIT: I just ordered it since I just noticed that I had a $25 worth of rewards points on my Amazon Visa card...

I guess it's all relative. IE: If I was suddenly blessed with getting as much 1/2 inch as you did,  adding a $150+ dollars to the equation would be a no brainer.

 

Congrats on scoring the cable!

[gman1971]

Lets not sugar coat this and get to the point:

 

I've had horrible experience using MPD made LMR400 cables. First off, these cables were a PIM galore, riddled with high SWR and noise, and windy day? forget to hear anything but noise due to the cables swinging in the wind.. In addition, the crimping always failed too. Connectors are chrome plated cheap garbage, even though they claim silver, they are NOT silver. The crimping is not weatherproof either, they are not using anything to help weatherproofing at all, its just a regular POS shrink tubing... nada. So, when sitting outside the outer jacket will rust and the already crappy LMR400 PIM-galore performance will turn into utter trash with sky high SWRs.

 

For a 100 feet run, please, save yourself all the headaches and buy Heliax, 1/2" should be a good start; seriously, don't bother with LMR garbage. I can hear it... "here comes another heliax snob..." yes yes, I've heard that before, but let say I owned a ton of LMR400 too, I always thought it was the best thing since sliced bread...., I thought it was great... ha... ha... ha.... until I tried Heliax and chrome-less connectors... never ever ever in a million years will use LMR for anything longer than a 3-4 feet patch, and silver plated connectors or tri-metal ONLY.

 

In addition to that, I will reiterate what everyone here has already stated: use N connectors, preferably Commscope tri-metal ones for the long Heliax runs; the cheapo chrome plated ones suck at impedance as I've found out. If you want performance, be prepared to spend $$$, unless you live atop Mount Everest...

 

Yes, its hard to swap the UHF thingy, but I've recently done it, and it was well worth it, and once you have the setup running you'll love it: no more sealing connectors, ever, no more creeping SWR after a couple of thunderstorms... no more hearing the NOAA due to PIM garbage LMR400 swaying in the wind... etc..

 

G.

 

EDIT: Oh, don't forget to ground PROPERLY the antenna/mast/radio to the same ground rod. Massive improvement when I did this in noise floor threshold reduction...

[Riktar]

Thanks for the tips. If I go over the 100 foot mark by moving the antenna out of the attic and onto the roof I will definately look into the Heliax. For now I am going to stay with my 55 foot run into the attic. I originally planned on a 90 foot run using a 15 foot pipe next to the house but went with an attic mount solution to avoid outdoor elements and allow me to shrink my cable length needs. Was a good thing too. I had enough extra to cable to allow a couple of "whoopsies" with my initial attempts at using crimp connectors on the LMR400. 

 

Along the line of grounding: Since I am mounted on an attic rafter, I don't have any metal contacting the antenna at all. I don't suppose I could just run a ground line to the plumbing vent in the attic? Run a grounding line down to the basement and ground to the well pipe coming into the building? 

 

Thanks again for the advice!

[gman1971]

Well, attic and indoor certainly helps with SO239 and crimped cables rusting outdoors... but LMR400 in general is just too hyped. Honestly I would ditch it and get Heliax. You can find surplus heliax quite affordable.... but if you must have LMR400 at least use silver plated solderless N connectors like these:

https://www.ebay.com/itm/N-Male-Coax-Connector-Silver-Clamp-LMR-400-9913-by-W5SWL/221584358558?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649. I have a small 4 feet patch between the top arrester and the antenna, the N side is using such connector...  and the other side is using a 100% silver plated UHF connector, sold from the same guy. (since antenna is UHF...) next antenna will be N connector... and hopefully tri-metal too... we'll see.

 

Again, I strongly advice to ditch the SO239 as early as possible in your hobby endeavor. Otherwise you'll have to redo everything at a cost later...

 

As for grounding, sure, the metal mast I am using is mounted on the roof with no grounded metal nearby either so I ran a solid 10AWG copper wire to get it grounded directly to the rod at the electrical box. Ideally you'd want to ground yours to the outside ground rod too, and use the same cable for grounding all stuff... but any metal pipe that is grounded should work, just make sure its grounded, tho. I would still ground everything to the same ground wire if possible.

 

I've found out that the more I improved the grounding on the mast/antenna/radio the more it made cable lengths less important for SWR. When I had the antenna 100% ungrounded, then using different cable lengths gave vastly different SWR readings at the radio end, but once I started grounding it, the SWR differences between lengths became much smaller.

 

The explanation I've read for this is that verticals tend to have issues with feed line decoupling (I am using a TRAM 1450 vertical for GMRS) whereas higher quality antennas like the DB404 folded dipoles tend to not have this kind of issues due to how folded dipoles work.

 

Arresters, I think they are great for grounding the feedline, and I suggest using two, even for an attic setup like yours. One placed right under the antenna and another somewhere closer to the radio. I've had very good results with these Times Microwave LP-BTR-NFF, which are fairly inexpensive and use tri-metal instead of chrome plated connectors.

 

Lastly, I also recommend coiling two or three times the patch cable going from the top arrester to the antenna with around 8 inches diameter in each turn. That in conjunction with a sold ground should improve noise threshold quite a bit. Having the arresters at each end of the mast also made the installation really manageable, since now I can bring the antenna(s) down without having to remove or deal with the 20 feet Heliax anaconda feedline...

 

 

G.

 

Thanks for the tips. If I go over the 100 foot mark by moving the antenna out of the attic and onto the roof I will definately look into the Heliax. For now I am going to stay with my 55 foot run into the attic. I originally planned on a 90 foot run using a 15 foot pipe next to the house but went with an attic mount solution to avoid outdoor elements and allow me to shrink my cable length needs. Was a good thing too. I had enough extra to cable to allow a couple of "whoopsies" with my initial attempts at using crimp connectors on the LMR400. 

 

Along the line of grounding: Since I am mounted on an attic rafter, I don't have any metal contacting the antenna at all. I don't suppose I could just run a ground line to the plumbing vent in the attic? Run a grounding line down to the basement and ground to the well pipe coming into the building? 

 

Thanks again for the advice!

[Riktar]

I should have mentioned the antenna connection is a N type. As to the PL-259 connections the only one is the connection to the back of the Kenwood since I am trying to have only one line (In my case the LMR400) going from the antenna to the radio.

 

Now I am sure this is a real case of overthinking but would there be any benefit to removing the SO-239 on the radio and installing a Female N-connector so I could utilize Male N connections on both ends of the cable?

 

I will get on the grounding stuff this weekend. Shame I don't have the nerve to tackle the roof mounting. There are lightning arrestors up by the chimneys with cabling you could use to pull a tank with, going down to grounding rods on two of the corners of the house. Maybe the Son-in_law will be up this weekend.....     

[gman1971]

Very cool, yeah N connectors are great if only b/c they are sealed... which is great for outdoor installations. Also I think there are more options out there for higher end gear with N than SO239...

 

Unless you're super skilled in soldering this kind of stuff I wouldn't attempt replacing the SO-239 on the back of the radio...  and I've thought about it too. Just not worth the ris

 

All my Vertex Standard mobile radios use mini-UHF which are chrome plated and a pain in the rear to work with...etc... Had to make patch cables from mini UHF to N...  

 

The radio UHF connector is probably the least of the worries, as the radio usually is not sitting outside taking water... etc, the real problem arises when you start going down the UHF route b/c its what inexpensive gear comes with...  after some time you realize you have 10 devices that all use the UHF and changing course is just hard as heck.

 

I've had good luck using Deoxit to clean and lube the So239 connectors prior to connecting it...

 

G.

 

I should have mentioned the antenna connection is a N type. As to the PL-259 connections the only one is the connection to the back of the Kenwood since I am trying to have only one line (In my case the LMR400) going from the antenna to the radio.

 

Now I am sure this is a real case of overthinking but would there be any benefit to removing the SA-239 on the radio and installing a Female N-connector so I could utilize Male N connections on both ends of the cable?

 

I will get on the grounding stuff this weekend. Shame I don't have the nerve to tackle the roof mounting. There are lightning arrestors up by the chimneys with cabling you could use to pull a tank with, going down to grounding rods on two of the corners of the house. Maybe the Son-in_law will be up this weekend.....     

[Jones]

Just an insert note here for those new-to-the-hobby folks who are totally confused by the last few posts... "UHF" connectors, as they have been called for decades, are not really good for use on what we now know as the UHF band.  Back in the old days, UHF was considered pretty much anything over 30MHz.

 

Now, what's all this talk about SO and PL connectors?  The military designation "SO-239" is the female UHF connector.  The "PL-259" is the male UHF plug. These are the antenna connectors that have been used on CB radios since the 1960s, and still are today. You will also see reference to "SO-238" or "PL-258", which is a double female "Barrel" connector for hooking two coax cable together if both have PL-259 plugs. Those should be avoided at all cost on GMRS. If you need a longer coax, buy a longer coax, don't try to extend one.

 

Although UHF connectors are fine for CB, they just aren't as good on GMRS.  Constant impedance, and weather protected type N connectors are much preferred.

[marcspaz]

Agreed... SMA would be my preferred connectors