Home-Brew 2m mobile antenna for fiberglass body RV

Discussion in 'Mobile Radio Systems' started by KK6VLV, May 30, 2016.

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  1. KK6VLV

    KK6VLV Ham Member QRZ Page

    It all started when my Yaesu 857D had to go back to the factory for warranty repair.

    Because I live in a somewhat low area surrounded by ridges in most directions I needed radio that would have a lot more power than HT while the Yaesu is being repaired.

    I also wanted a mobile radio that I could mount in the cab portion of the RV and also considering that my budget is very, and the fact that I did not want to worry too much about how much it would set me back and hurt my feelings if somebody stole it, so it seemed to me that a good quality mono band mobile 2m radio would be just right, so I got myself a ICOM 2300H that was on discount at the time from HRO.

    Enough said about the radio now the actual topic of the 2m home-brew antenna that I decided to build for my fiberglass body RV.

    I decided that the best way to get a reasonably good quality gain antenna on my fiberglass body to RV was to build my own, so I decided to look into what my options actually were.

    After deciding that number one;

    I do not want to drill any holes or add any metal to the fiberglass roof ,

    number two; I do not want any antennas in my forward driving field of view;

    number three, no magnetic mount (I already have one of those that I don't like it that much, and I would have to violate rule number two to use the magnetic mount antenna anyway),

    number four, I do not want to mount it either to close to the passenger seat, or me, the driver because the radio RF is 65 Watts, and I like to rag-chew, (although usually not while driving, but while I'm parked but still sitting in one of the seats is another story), therefore the fender and hood areas are out.

    So, A; HF Yaesu radio was on its way to the shop for repair, and B, I decided, because I did not have that much positive results with the HF bug catcher type mobile antenna that I built a few months ago, that the mounting bracket that already existed on the left rear bumper would be a good start to build up from.

    Therefore, (C), I realize that it had to have its own ground plane circuit so to speak, which would be independent from the RV because after all, the RV is mostly fiberglass and there is insufficient ground plane for reasonably well dispersed signal pattern metallic parts that do exist in that area.

    I needed to come up with something that did not stick up so high that it would hit solid objects like trees, etc. and likely get broken from hitting the more solid objects such as large branches, etc..

    I looked up several variants of the J pole like the Arrow (way too expensive to buy and build, mounting issues etc.), ribbon-line 2m/70cm J-pole (may not handle the 65 Watts), and the 1/2" copper pipe J-pole ( too cumbersome and would bend and break too easily; not likely to handle highway speed and strong headwind in mobile application, low trees would be issue too).

    I ended up with an antenna design that could be used as base station antenna which I had been experimenting with before the Yaesu went on the fritz... ...I have already built one o them for 2m and 70cm dual band for my base/portable station (It worked fairly well), so I was familiar with it.

    The design I used is from this web page:

    I read up on this design and have already had some issues with RF in the shack with other antennas I have used in this fiberglass RV and came to realize that I needed to use some kind of RF choke at the antenna feed-point.

    After doing some research I ended up getting 2 different values of ferrite beads from Amidon (FB-43-5622 and FB-43-5623).

    I used 4 of the FB-43-5622 between the (2) FB-43-5622 (one at each end of the whole string of beads near the antenna's feed-point) using one single length of Times Microwave LMR-240 Ultra-Flex coax from the radio to the antenna; and is soldered to the antenna directly (no connectors @ antenna).

    I estimate the string of 6 (total) beads as strung @ the feed-point is ~ 300 Ohms @ 2m band average.

    Once it had SWR where I wanted it tuned for FM repeater and FM simplex frequencies (centered @> 146.0 MHz) I sealed the soldered feed-point with liquid electrical tape, then painted it with self-priming paint that sticks to plastic (the wire and PVC pipe are plastic).

    I then used clear Flex Seal to seal the antenna, I also coated the beads with it and then used vinyl electrical tape to further stabilize and protect the ferrite beads.

    I also put a shorter string of ferrite beads at the radio end of the length of coax about 4" from the PL-259 connector.

    SWR is below 1.5:1 (~ 1.35:1 maximum) on all repeater input frequencies (USA).

    Attached Files:

  2. KK6VLV

    KK6VLV Ham Member QRZ Page

    :pwoops!! I said: "(the wire and PVC pipe are plastic)... ...
    I should have said: (the wire's outer insulation and PVC pipe are plastic):D
  3. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Slim Jims as they're called, J-Poles, and similar designed antennas will always have a fair amount of common mode—the main RFI source in these cases. As you've discovered, you have to choke off the common mode. But you didn't need to use 6 beads. All you needed is one! Remember, the amount of reactance in an inductor is a square function. So just three turns through one bead, and you'd have 450Ω (assuming your calculation was correct). Better yet, the resistive comment of the choke will be higher than the inductive reactance in ohms, and this is exactly what you want in a choke. This is NOT the case of the one you're using—six, single turn chokes.
  4. KK6VLV

    KK6VLV Ham Member QRZ Page

    Thanks for your input.

    I did consider using the thread-through larger diameter bead for the 'square function" as you mentioned.

    For the mobile and me not wanting to stress the foam type insulation of the coax with the turns, and it being exposed to direct desert sunlight (I travel to the desert sometimes) and 'relatively' high heat and motion exposure etc., I think the series resistance of the beads here is likely to be enough for ~ 100 Watts;, (I am guessing).

    The series impedance method is less likely to present mechanical problems (IMHO) and takes less space on this mini RV.
    The available space is limited.

    I plan for other antenna projects and I will be using more coax junctions in that area to utilize for home and 'stationary' portable use of other antennas.

    Since I only have 65 Watts RF 2m radio connected to it, I believe I am OK with what I have as installed.

    When I am stationary (at home or camping, etc.) I oftentimes take the 'mobile' antenna out of the mount and bungee it to the grab handle and not use the mounted in the cab radio in the RV; I then then use the Yaesu 857 radio on the dual band 'base' station Slim-Jim antenna at a higher ATG position for 2m and 70cm.

    What Amidon states about the FB-43-5623 Beads:

    Product Description:

    Suppresses 25 MHz to 300 MHz. Dimensions are in inches. Some typical impedances are listed for each part.

    ID: 0.250

    OD: 0.562

    Height: 0.530

    Ω @10 MHz: 51

    Ω @25 MHz: 78

    Ω @100 MHz: 118

    Ω @250 MHz: 140"

    And for the 43-5622 Beads:

    "ID: 0.250

    OD: 0.562

    Height: 0.400

    Ω @10 MHz: 39

    Ω @25 MHz: 61

    Ω @100 MHz: 89

    Ω @250 MHz: 104"


  5. KC9ZHR

    KC9ZHR Ham Member QRZ Page

    I am usually not as allergic to jpoles as some but given your height above the cab there I ould probobly just make a groundplane that sticks out just above the roof where you have the slim jim and be done with it.....It would probobly perform better as well since the high current point will be higher than it is now but you would still have the same clearance you have now.
  6. KK6VLV

    KK6VLV Ham Member QRZ Page

    I finally found (built) an antenna set up that has worked very well for me.
    In a previous post I did mention the "Arrow" antenna. In that post I stated that it was too expensive for me to buy or even build; I was wrong about that, building it was not too expensive at all.

    I found a place that sells metal inexpensively enough that it was well worth buying the aluminum rod and angle iron, at the minimum length they would sell. I was even able to afford to get tap and die set that had the 3/8" x 24 threads recommended for the build.

    The mod for mobile use was to chop-off as much of the 57 1/2 inch length aluminum rod and somehow attach a small diameter stainless spring steel mobile whip without changing the SWR impedance values too much.

    2B_2m-70cm Arrow J Pole_with SS whip on fiberglass body Mini RV_rs_edit_05_26_2017.jpg

    Antenna Analyzer Results For 2m:
    144.049 SWR= 1.2, Impedance = 43 Ohm, X= 6
    146.032 SWR= 1.2, Impedance = 56 Ohm, X= 12
    147.997 SWR= 1.4, Impedance = 52 Ohm, X= 18

    The antenna SWR impedance values for 70cm are not nearly as good as it is for 2m, but, since I only have a 2m radio in the mobile, it doesn't matter much to me at the moment. It may be adjustable by changing the length of the 70cm stub. I just have not had the need to go on with it any further. 2m is what I was trying to accomplish.

    Antenna Analyzer Results For 70cm:
    432.345 SWR= 1.7, Impedance = 69 Ohm
    436..890 SWR= 2.1, Impedance = 41 Ohm
    440.548 SWR= 1.5, Impedance = 39 Ohm
    445.195 SWR= 1,8, Impedance = 60 Ohm
    449.867 SWR= 1.6, Impedance = 32 Ohm

    Even though it raised the overall cost considerably, I decided under my circumstances that static buildup discharges strong enough to be potentially damaging to the radio were more likely than not.
    Anyway, the next paragraph is a snippet of the article that I think is an important note of something I thought I should seriously consider:

    "The 'Arrow' antenna is NOT a DC 'Grounded' antenna!
    Because of that, I decided to use a PolyPhaser Coaxial Lightning Protector model IS-50UXC0 just before the coax gets to the Radio because I regularly travel and stay in the very dry desert where static discharges are more common on a daily basis than not."

    I wrote a more detailed description of it on it that explains it far more than I have here.
    It was published on a local ham radio club's website (w6mmm.club), The URL to the article is:

    I will eventually build a properly constructed entry enclosure with a copper ground plate and ground strap that can be connected to a ground rod for some improvement on static discharges and some minimal lightning protection and for when the RV is connected to shore power etc...

    73' From: KK6VLV

    Last edited: Jun 30, 2018
  7. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    You did a lot of things which weren't necessary.

    Everyone who owns an antenna analyzer should buy Joel Hallas' (W1ZR) booklet, Understanding Your Antenna Analyzer. Nice to know you have a reasonable SWR, but that fact typically ISN'T the most important component.

    Crazy baluns (coiled coax) have VERY low common mode rejection, and poor bandwidth. In this case, a single, 1/4 inch ID, Mix 31 split bead is all you need.

    A simple 1/4 wave made like the one in the link, will probably work better. If you use a piece of 1/4 copper tubing for the vertical element instead of wire, The antenna will cover all of the 450 MHz FM band with a decent SWR. The element will end up being a bit shorter than the formula due to its larger OD. http://www.ccars.org/projects/2mGP/tech_2mgp.htm

    I don't fault folks for using lightning protection (I've been hit three times while underway). A Polyphaser may help to protect the transceiver, but don't rely on it doing so. And it you get a direct hit, you'll need more than a new transceiver... Say new tires!
  8. KK6VLV

    KK6VLV Ham Member QRZ Page

    Thanks, I will keep in mind your suggestion of 31 mix split bead for when I upgrade the run of coax from front of RV to rear from the LMR 240-UF to RG-8 (stranded center conductor).

    Agreed. My mobile is single band 2m ICOM 2300H, so no emphasis was put into the 70cm band for the mod.

    I will eventually consider a separate 70cm 1/4 wave GP for future, maybe for when in parked portable mode or if I figure out a robust design, and good location on RV for it and a way to mount it there.

    I do admit there are likely better mounting arrangements that I could consider than the one I am using now. For now it, as tested, with analyzer and on the air, is quite adequate at tx and rx. Also, it is far more resilient to the low tree limbs as mentioned in the article than it was before the mod which was my main goal in doing the mod.

    My primary reasoning for the PolyPhaser, as I stated in the article I linked to...
    ..."because I regularly travel and stay in the very dry
    desert where static discharges are more common on a daily basis than not."

    ...I think that the PolyPhaser unit for the non-DC grounded antenna is better than no static discharge dissipation of any kind in the antenna system... ...In that sense I am 'relying' on it to mitigate at least to some extent 'ESD" from at least some amount atmospheric 'Triboelectric effect'.
    As for 'relying' on it for protection from lightning strikes?... ...Not really; It is not even mounted in a proper location for lightning protection.

    Just did something that is better than doing nothing for protecting against 'ESD' from wind, and dust storm friction; nothing more, nothing less would be reasonable to 'rely' upon it to do as is currently configured.

    That being said, I still think there is, albeit would be minimal to the extreme end of 'minimal', some 'protection' from atmospheric condition caused by thunderstorm activity, (or other atmospheric phenomena) causing a "Triboelectric effect" that could potentially cause electrical imbalance enough to cause potentially damaging 'ESD' between the center coax connection at the antenna feed-point and 'vehicle-grounded' coax braid.
    I don't know if My explanation would pass any grade in electrical engineering class, but, that is my reasoning at this point with my minimal (yes maybe "minimal to the extreme end of 'minimal'":D) 'knowledge'.o_O

  9. K7JEM

    K7JEM Ham Member QRZ Page

    You could install a shorted quarter wave stub on the feedline, it would DC ground the center conductor, yet be invisible at RF.
  10. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, the Gas discharge devices don't fire at a low enough voltage to take care of dust and tire (rolling) static. The thing you try to do, is discharge it BEFORE it causes a problem. DC ground antenna help, but a simple 100kΩ resistor across the antenna terminals works just as well. And don't forget a static drain. Info on that is on my web site.

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