[AK-VHF] AK-VHF Net: 29 Jan - 146.52 FM

Tue Feb 1 19:50:47 EST 2022

Hi Ed,

That's a good point about yagi antennas. For a while now I've gone back and forth on the idea of having a VHF/UHF yagi for on foot use. The arrow antenna models are well built and effective, but a little too bulky for my taste (in a backpack-portable context anyway). A few years ago I used 4NEC2 to design a really simple, wideband, 3 element yagi for SOTA use. It is based around a 1/2" diameter PVC pipe mast, and uses 3/32" bronze brazing rod elements. The elements would slip out and store inside the mast for travel. I never built it though.

With Yagis in the field I think the question is always whether the added aiming complexity outweighs the improvements in Tx/Rx strength. I once used a little 4-element yagi to work FM from Fremont Peak (at the base) back to my home town of Menifee, back in Ca. At about 115 miles I was pretty happy with the results, but it can definitely be a pain to get the antenna up in the air.

What I've found since moving to Alaska though is that the trees are the limiting factor for most VHF/UHF work. Using roving as an example, when I go out I find that one of two cases usually arises:
1. I'm close enough to other stations, or high up enough, to the point where a yagi isn't needed. (Point Woronzof, Glen Alps, Diamond Ridge Road)
2. I'm far out enough that there are miles of forest in the way, and without a 50' mast on the truck I can't reach anyone. (Having the yagis at truck height in the forest doesn't give any advantages.)

There are some places where it's a definite advantage, like activating BP50 from Hope or Indian, but for most portable sites, usually #1 or #2 above ends up being the case.

For a home station it's different: with the yagis on top of the house (40' or thereabouts) I can reach a LOT more stations on simplex, and the gain really helps the signal to noise ratio. Out in the forest though . . . can't see the signals for the trees.

Brandon

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-------- Original Message --------
On Feb 1, 2022, 13:29, Ed Cole wrote:

> Yes, considered hauling around 5-foot mast section on foot would be awkward.
>
> one more idea for VHF/UHF: Are familiar with the Arrow antenna. Its a
> dual yagi with 3-elements for 2m at right angles to 6-elements for 70cm.
> It has a low power diplexer if using with radio that has only one coax
> connector, but one can connect coax separately by BNC.
>
> The antenna is made of aluminum arrow shafts for elements that screw
> into the rectangular boom (also two piece). Dissembled the boom and
> elements fit nicely into a small (two-foot long) pvc pipe with end caps.
>
> I have one made up for my arrow with holes in end caps to insert a
> 1/4-inch rope which serves as handle or for looping over one's shoulder
> for carrying.
>
> Good part is you have decent gain on both bands. It comes with a rubber
> grip to hold by back end of boom.
>
> Only challenge in backpack mode would be having a mast to get the
> antenna above brush height. Maybe scrounge a stick one could use with a
> U-clamp to the boom?
>
> Ed
> blowing snow- indoor day!
>
> On 2/1/2022 12:48 PM, Brandon Clark via ak-vhf wrote:
>> Yep, definitely had the picture (map?) rotated a bit in my mind.
>>
>> J pole antennas have a lot of advantages: not too large, vertical polarization, a little bit of gain, easy coax routing out the bottom, and no need for a ground plane. My first 2 meter antenna was a copper plumbing pipe j pole that I built. I initially had the coax center and shield connected to the metal with hose clamps, but later on soldered one of those flange connectors you mentioned as a replacement. With the 1/2" diameter tubing the < 1.5:1 SWR bandwidth covers the whole 2 M band.
>>
>> I didn't use that antenna much over the last few years, but recently put it back into service again as my POTA VHF antenna. It works great when I'm activating from my truck; I used some leftover fiberglass box tubing scraps (McMaster Carr) to make an insulated mount that connects the antenna to the cross bars on my camper shell. Clean, simple, sturdy. That antenna is a bit long and cumbersome to carry into the backcountry though.
>>
>> What I've been using for backpack-portable work is a fan dipole utilizing bronze brazing rods for the elements, connected to a simple center insulator I made from some leftover plastic and machine screws. Ring terminals soldered onto the brazing rods make for easy connection to the center insulator using wing nuts on the machine screws. With brazing rod element pairs for both 2 M and 70 cm I get a nice clean pattern on both bands. A collapsible tent pole makes for a backpack portable mast.
>>
>> The only drawback is that the antenna and tent pole are a little bulky on a backpack even when collapsed and disassembled. I'm thinking to do the same general idea, but utilize the leftover window line to make the elements, and only bring the tent pole when I don't have another means of support (tree, another antenna, etc.). With the ladder line I can cut the sides for different band combinations and bring whatever I want to run: 2 M, 6 M, 70 cm, or other bands. In a pinch it could even serve as a backup HF antenna (with some other longer scrounged wire and my tuner).
>>
>> Brandon Clark, KL7BSC
>> https://brandonclarklabs.com/
>>
>> Sent with [ProtonMail](https://protonmail.com/) Secure Email.
>>
>> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
>>
>> On Tuesday, February 1st, 2022 at 11:25, Ed Cole kl7uw at acsalaska.net wrote:
>>
>>> Brandon,
>>>
>>> OK, then wrong picture in head?
>>>
>>> I had a 80/40m alligator clip dipole for use on the Iditarod (even
>>>
>>> called Iditarod Special). Was simple, pretuned, roll-up portable.
>>>
>>> My 80/40m inverted-V is fan-style. But I found 40m to be very narrow
>>>
>>> band probably due to influence of the 80m wires. Better to spread each
>>>
>>> band out horizontal several feet. problem for portable is having end
>>>
>>> supports for the antenna.
>>>
>>> When I rebuild my dipole 40m wires will be 90 degrees from 80m wires and
>>>
>>> be fed with open wire from ground where my 1:1 balun will connect for
>>>
>>> 80/40m. I will short the open wire and connect a base coil for loading
>>>
>>> as a loaded vertical on 630 or 160m. Of course not a portable antenna.
>>>
>>> Instead of twin lead j-pole consider a metal tubing j-pole. Can be made
>>>
>>> from 1/8 or 1/4 brass tubing and soldered with coax flange connector to
>>>
>>> attach coax to. Have a 2-inch mailing tube or maybe pvc pipe with caps
>>>
>>> for carrying.
>>>
>>> I even made a J-pole using TV-mast as main part and adding a 1/4 wave
>>>
>>> stub of 1/2 inch copper to the side 1/2 wave below the top. Just
>>>
>>> connected coax at bottom of stub and mast at the same point (stub
>>>
>>> insulated from mast). That was my first 2m antenna when I lived in
>>>
>>> Hope. Worked 94 repeater FB.
>>>
>>> 73, Ed
>>>
>>> On 2/1/2022 10:01 AM, Brandon Clark via ak-vhf wrote:
>>>
>>>> Hi Ed,
>>>>
>>>> The funny part is that I can't even use the "I didn't have a compass" excuse. I always bring one, but I didn't use it in locating my preferred campsite since I knew I was at the right lake, so had a good picture of things in my head . . . or so I thought, lol.
>>>>
>>>> The kind of linked dipole you mention (with the alligator clips) work great. One of those cut for 40 M and 20 M used to be my go-to antenna for summits on the air back in Ca. With a telescoping fiberglass mast from MFJ as a support it worked great.
>>>>
>>>> The j pole that I built (which had the problem) was one of the "DBJ-2" designs that are really popular. It is made from twin lead line with a small shorting section of RG-174 in the radiator, allowing it to perform with a good pattern on the 70 cm band. (The shorting RG-174 is inductive on the 2 M band and so just shortens the radiator a bit.) I've built one or two before, and have always found them to be tricky to build, and, frankly, a bit delicate.
>>>>
>>>> Electrically they are a great design. The DBJ-1 was intended to be mounted inside a PVC tube and used as a base station antenna, for which it works great. The DBJ-2 forgoes the PVC tube and is intended for portable and emergency use as a roll-up antenna. They always just seem really delicate though, and I suspect something broke on mine when exposed to the cold.
>>>>
>>>> I have in mind a new design for a portable multi-band fan dipole VHF antenna that I'll be building soon. Super simple, cheap, a roll-up design, and it will even include the 6 meter band. I'm hoping to get it built this week, so I'll send out info when I get it working.
>>>>
>>>> 73,
>>>>
>>>> Brandon Clark, KL7BSC
>>>>
>>>> https://brandonclarklabs.com/
>>>>
>>>> Sent with ProtonMail Secure Email.
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