Resonate Feedline Antenna

Modeled, Built and Presented

By:

Steve Fletcher

N5BOY

5-15-2006

VHF Resonant Feedline VHF Resonant Feedline DipoleDipole

Common VHF Antennas Common VHF Antennas Modeled And ComparedModeled And Compared

Groundplane J-Pole Arrow type dual band groundplane Dipole Resonant feedline dipole

IntroductionIntroduction

2 Meter Groundplane, Sloping Radials 10 Meters High

Electric Units & Ohms LawElectric Units & Ohms Law

2 Meter J-Pole 10 Meters High

Conductors, Insulators & Conductors, Insulators & CurrentsCurrents

2 Meter J Pole RF Current

FrequencyFrequency

2 / 70 cm Meter Arrow Groundplane 10 Meters High

Fundamental and Fundamental and 2nd & 3rdHarmonic 2nd & 3rdHarmonic

2 Meter Dipole 10 Meters High

2 Meter Dipole RF Current

Electrical AnalogiesElectrical Analogies

2 Meter Resonant Feedline Dipole 10 Meters High

Circuit DefinitionsCircuit Definitions

2 Meter Resonant Feedline Dipole RF Current

Series & Parallel AdditionSeries & Parallel Addition

2 Meter Groundplane with Sloping Radials 1 Meter High

DC MeasurementsDC Measurements

2 Meter J-Pole 1 Meter High

Circuit MeasurementCircuit Measurement

2 / 70 cm Meter Groundplane, Arrow Type 1 Meter High

Harmonic DistortionHarmonic Distortion

2 Meter Dipole 1 Meter High

Radio Frequency Filters Radio Frequency Filters

2 Meter Resonant Feedline Dipole 1 Meter High

Height Above Ground: 10 Meters 1 Meter------------------------------------------------------------------- Groundplane6.39 1.78 J-Pole 7.18 2.15

Dipole 7.60 7.38 Res. Fl. Dipole 7.57 7.36 Arrow GP2 / 70 cm 6.39 / 4.61 1.78 / 5.59 5/8 wave GP 7.46 5.88

* Gain in db compared to an isotropic radiator in free space.

While all of the antennas modeled have similar performance at a height of 10 meters, the dipole performs much better when used in close proximity to the ground. This is an important consideration when designing an antenna for use inside a shelter at table top height.

Advantages of the Resonant Feedline DipoleAdvantages of the Resonant Feedline Dipole Maintains gain at very low (table top) height. Less sensitive to surroundings than a J-Pole. Low feedline radiation is intrinsic to the design. Easier to tune than a J-Pole. SWR changes very little when position or height is changed. Shorter than the J-Pole. More narrow than a groundplane, no radials to dodge at table

top height. Easier to store and transport than a groundplane. No tuning stubs exposed to the environment. Easy to mount. Very cheap to build. Becomes a duel band antenna when topped with a duel band

radiator. More gain at any height than a groundplane or J-Pole. Packs away in the trunk of even the smallest car taking

almost no space.

Building a Resonant Feedline Building a Resonant Feedline Dipole for 2 MetersDipole for 2 Meters

By: N5BOYBy: N5BOY

Material3/4 Inch PVC

pipe

Cut dimension A to B

23 or more inches

A

B

C

D

Measure 16 Inches from the top of the pipe and mark

with a pencil or pen

Drill a hole at the 16 Inch Mark. Make the hole big

enough for a piece of RG58 coax to pass through with

out binding.

After the drill bit passes into the pipe turn the drill

so that the tip of the bit points to the top of the pipe.

This will bevel the top inside and bottom outside of the hole to prevent the sharp edges of the hole

from cutting into the coax

Solder the center conductor and shield of your coax to the SO239 connector.Seal the coax, especially the braid with hot glue, silicone, or liquid electrical tape. This keeps stray “hairs” from the braid from shorting out the center conductor and also keeps moisture out of the braid should you find yourself using the antenna outside.Use solid dielectric RG58. Coax with a foam dielectric will NOT work. Using larger coax will make it hard to form the choke.

Prepare a ¾ inch pipe cap by drilling a hole in the center of the cap. The hole should be large enough to snugly mount the SO239.A flat cap is preferred but a round top cap may be used if the top is ground flat and thinned out enough to allow the SO239 to be mounted properly.

Mount the SO239 in the pipe cap and tighten the mounting nut.

Thread the free end of the RG58 coax through the top of the pipe and bring it out of the hole you drilled at the 16 inch mark. Actually it would be easier to thread the coax through the hole and out the top before you soldier the SO239 on the coax. Either way will work.

Pull the slack out of the cable and secure the cap on top of the pipe. It is usually not necessary to glue the cap

Pull the coax snug and secure it with a plastic cable tie.

Form the choke by winding 9 turns of coax, snug against the pipe with no gap between the turns.

Secure the bottom of the coil with a cable tie. To help secure the coax put another cable tie an inch

or two below the tie at the bottom of the choke.

The choke may be covered, secured and weather proofed two ways.

A) Tightly cover the choke and coax from two inches above and below with a good quality electrical tape. Tightly stretch the tape so that it will form to the choke.

B) Cover it with heat shrink tubing.

Last step is to put a PL259 on the end of the coax.

Finished antenna bottom section on a pipe flange stand.

Building a ¼ Wave Top SectionBuilding a ¼ Wave Top Section

Solder a 20 inch piece of welding rod or any stiff wire (coat hanger) to the center of a PL259. Set up the antenna about 5 feet off of the ground and trim the top of the wire until your lowest SWR is around 146.500. You should have no problem getting it down to 1.2:1 with SWR no higher than 1.5:1 across the entire 2 meter band. Fill the hole in the top of the PL259 with hot glue, silicone, or liquid electrical ape.

Newbie Tip Remember if the best SWR is at the low end of the band, the

antenna is too long. If the best SWR is at the high end of the band the antenna is too short. Star off with a wire you know is too long and trim a little at a time, it is very easy to cut off wire but very hard to ADD wire.

A loop in the top of the antenna will give you a way to hang it if you so desire and it will make the antenna a little more broad banded. To put a loop on your antenna first tune it to the lower end of the band, lowest SWR at about 144.500 Mhz. Now bend a loop in the top of the antenna. Bending the loop effectively shortens the antenna and should bring the point of best SWR up to something close to 146.00 Mhz. SWR at the top and bottom of the band is usually a point or so better with the loop on top.

Other Ways to Top Your Other Ways to Top Your AntennaAntenna

With a BNC adapter you may use any HT antenna on top of the resonant shield base. The telescoping 5/8 wave antenna will give you some gain. The rubber duck will work better on the resonant shield base than it does on the HT but it will still radiate like a dummy load. Maybe more like a leaky dummy load.A mobile antenna with a PL259 type mount may also be used on the resonant shield base.

Variations on a ThemeVariations on a Theme

A rollup version made by stripping 19 inches of shield off the coax and winding a choke on a piece of scrap PVC. Trim the center conductor for best SWR and add a loop so that the antenna can be hung from a tree limb.

No drilling of the PVC pipe necessary for this version.

The whip slips neatly into the base for storage.

A PVC pipe cap keeps the whip inside.WARNING: If you use an antenna other than the ¼ wave whip, do NOT leave the whip inside the base.

Why I Reinvented the WheelWhy I Reinvented the Wheel In the days immediately after hurricane Rita, we had virtually no repeaters

operational in South Texas ARES district 9. We communicated by HF or 2 meter simplex. We found out the hard way that repeaters tend to suffer the same fate as cell phone towers. After life returned somewhat to normal, our local ARES / RACES members began discussing what we could do to maintain communications in the event of total repeater failure. In Hardin County we decided to use 2 meter simplex for our in county communications. We conducted a survey of our county and determined that we could maintain communications via 2 meter simplex with modest antennas.

This started Rich (N5ALE) and I searching for a solid bulletproof 2 meter antenna. The antenna had to be cheap, easy to build, and easy to transport in a very small car. We looked at, discussed, modeled, and tried many of the antenna designs available on the internet. We found a lot of designs that would work but we were not one hundred percent satisfied with any of them.

At this point I began thinking outside of the box. I started looking at old forgotten and novel ideas. In a 1957 handbook I found an interesting VHF antenna. The design used a metal sleeve over the coax to stop feedline radiation. This allowed the antenna to be fed from the bottom with no matching system. The design had definite advantages but I did not believe it would isolate the feedline well enough to fit our needs. I dismissed the sleeved antenna but it reminded me of a resonant feedline antenna for HF which I had seen in the 2004 handbook. I scaled the antenna down to 2 meter size and tried it out. The antenna modeled well, performed well in the real world and seemed to overcome many of the shortcomings we found in the more traditional designs. This antenna is not magic, it is a dipole, it has no gain but it does seem to perform better at table top level than the traditional Emcomm antennas we have used.

A number of Hams in District 9 built and started using this antenna after it proved itself in a real live ARES operation in Jefferson County. After heavy rains and flooding in Southeast Texas, the Red Cross opened a shelter. Jefferson County ARES EC Tootie Heintschel (KC5HVT) was asked to set up a ham radio station at the shelter. When a 5 watt HT and a mag mount antenna on a filling cabinet failed to break the local repeater the team decided to try Steve’s odd table top antenna. With the new antenna they were able to hit the repeater and maintain communications with the five watt HT. Yes the mag mount antenna was and is still in good working order.

If you want to try an antenna that is different, cheap and easy to build you might want to consider this design. 73’s Steve N5BOY

THE ENDTHE ENDSteve Fletcher (N5BOY)Steve Fletcher (N5BOY)