Antennas for QRP Operation

[vagrant]

Dusk hit just as I finished the transformer so I rolled out all of the wire, about 67' and did a quick sweep. I still need to add the pull lines and the chalk rollers. I used 25' of some BNC cable I had, so no choke yet for TX. I mainly wanted to see the numbers and wondered how much I would need to roll/cut on the end of the antenna wire, or if I had fudged something.

For now, it appears my single mistake was not drilling the hole for the antenna wire closer to the corner. That plastic box is more durable than I thought. In fact, I re-drilled closer to the corner and everything fit and did not get wonky. I now have a passive vent. Also, no silicone yet, as I was testing. I'll probably keep using the Velcro to close the case as tape leaves a residue.

The wire was only about eight feet off of the ground on each end with the following results. I obviously need to test with it elevated at different heights and see what's what. 40 meters was the most narrow, but the SWR was fine across the band. As you can observe on the sweep 20, 15 and 10 meters had more bandwidth. The numbers are the lowest for each band and mostly hit center on each.

I'll update once I test at different heights, both slope and horizontal configurations.


[ Click an image to enlarge ]

 

[prcguy]

Very nice! Can you post the VSWR over just 40m like 6 to 8MHz range? Looks like your wire length is about right with a dip at 7.16Mhz.

I can see you probably wound the transformer by sticking the end of the wire through the hole then pulling the rest through like threading a needle. That makes for lumpy windings. I found its best to do that with the very first winding of the primary then wrap the wire around the outside of the core then for the remaining turns push the wire already on the transformer through the hole first and pull the remaining length through after. That will make the wire hug the core and look nice.

I forgot to mention 10m will usually resonate higher near 29MHz and you can wind about 5 turns around a chunk of PVC pipe about 6 or 7ft from the transformer to help bring that down into the SSB region of 10m without affecting other bands. I'll look for an email I have from from Danny at MyAntennas that has the exact info. That is usually done on permanent installs but hard to do on a chalk line version.

Dusk hit just as I finished the transformer so I rolled out all of the wire, about 67' and did a quick sweep. I still need to add the pull lines and the chalk rollers. I used 25' of some BNC cable I had, so no choke yet for TX. I mainly wanted to see the numbers and wondered how much I would need to roll/cut on the end of the antenna wire, or if I had fudged something.

For now, it appears my single mistake was not drilling the hole for the antenna wire closer to the corner. That plastic box is more durable than I thought. In fact, I re-drilled closer to the corner and everything fit and did not get wonky. I now have a passive vent. Also, no silicone yet, as I was testing. I'll probably keep using the Velcro to close the case as tape leaves a residue.

The wire was only about eight feet off of the ground on each end with the following results. I obviously need to test with it elevated at different heights and see what's what. 40 meters was the most narrow, but the SWR was fine across the band. As you can observe on the sweep 20, 15 and 10 meters had more bandwidth. The numbers are the lowest for each band and mostly hit center on each.

I'll update once I test at different heights, both slope and horizontal configurations.


[ Click an image to enlarge ]
View attachment 77893View attachment 77894View attachment 77897

 

[N4GIX]

Interesting but how would you attach a guy string going opposite the wire to hold everything high in the air?

You could put a hangman's noose just behind the BNC female connector to provide the needed guy string.

This is an interesting thread. This may become a winter project for me prepare for next year's field day operations.

 

[vagrant]

I'm not sure I understand what you're saying about winding the core. I did the primary first then the rest. That first one after the two primary is wonky because I twisted it to the end using a drill. I had to untwist it and that's why it looks like that. I should have not gone all the way to the end and left two inches. We live we learn. I'm happy with the result for my first time doing that. Also happy to review a video of the correct way. I have observed some Youtube videos where their results looked horrendous, so...again happy to learn the correct method.

Here are the sweeps. I'm not sure why I didn't do this right off and show a finer detail. Anyways, now we can all enjoy the results. I adjusted the frequency edges to where they got near to 2.0 VSWR to provide a better range of the result, instead of just the upper and lower allocated band edges. After testing tomorrow, I may take photos again with the edges set to each band.

For those unfamiliar with this VNA, the number at the top middle in MHz is where the yellow marker is below. The numbers at the bottom left and right are the frequency edges.

 

[vagrant]

Whoops, I wired this wrong as I confused primary with secondary. I'll re-solder and post the new numbers later when time permits.

 

[prcguy]

VSWR looks real nice and just where I would want the freqs to land even though you got the hot and ground backwards at the connector. In fact the antenna should be perfectly usable wired backwards. I'm looking forward to a report comparing this antenna to other portable field types.

Whoops, I wired this wrong as I confused primary with secondary. I'll re-solder and post the new numbers later when time permits.

 

[vagrant]

I switched the wiring around on the transformer. The VSWR results are slightly higher now, but the wire is wet as it rained. Anyways, the rain is why I'm posting this as it provides results to compare and that it is still good to go when wet. Although, I may avoid 15 meters unless Japan is booming in. I haven't had a chance to test at different elevations, configurations yet.

As to comparing other antenna types, I expect it will beat the pants off of the loop, or the compromised verticals I have. I am interested in the results between it and the OCF.

( Never mind the 6 meter result )

Dry

Wet

 

[prcguy]

The VSWR results are just what I would expect, looks like its ready for prime time.

I think you will find it is much better than all the other antennas you mentioned and will be one of the easiest to put up and take down in the field. Its a real winner. An OCF of the same size will perform basically the same, the radiation patterns and gain would be nearly identical, but its hard to fit one in a chalk line reel with lugs for the balun.

An OCF is inherently unbalanced and will light up the coax with RF big time so it needs a good common mode choke. There are many ways to make the 4:1 balun and MyAntennas makes the best type but I don't have the recipe for that. You can make an excellent low power 4:1 balun by copying the kit sold by Elecraft where they use a small binocular core. It has excellent common mode rejection and covers 1 to 50MHz with great performance. Here is a link to the Elecraft balun and you can wind it with Teflon wire or even wire pulled from CAT-5 cable. https://ftp.elecraft.com/Mini Modules/Manuals Downloads/E740061 BL1 4 to 1 Rev B.pdf

The ferrite core for the Elecraft balun costs under $3 and I have its Amidon part # somewhere. I have a portable 133ft 80-10m OCF made with one of those that is a very good performer. A 133ft EFHW is also a great performer but you can't fit that much wire into a chalk line reel, so its a tossup between the EFHW and OCF if your looking for a good portable 80-10m portable QRP antenna.

I switched the wiring around on the transformer. The VSWR results are slightly higher now, but the wire is wet as it rained. Anyways, the rain is why I'm posting this as it provides results to compare and that it is still good to go when wet. Although, I may avoid 15 meters unless Japan is booming in. I haven't had a chance to test at different elevations, configurations yet.

As to comparing other antenna types, I expect it will beat the pants off of the loop, or the compromised verticals I have. I am interested in the results between it and the OCF.

( Never mind the 6 meter result )

Dry View attachment 77952

Wet View attachment 77953

 

[vagrant]

Agreed on the compact and convenience of this particular EFHW over an OCF of the same size. I have experience deploying a 40-10 OCF many times in various ways. It is why I planned to emulate this small thing, as well as gain build experience.

At home I have used a choke in PVC someone else made for about a decade at the feed point, but I did not notice any difference without it. This probably due to the LMR-400 coax run which is around 130 feet, mostly buried, and the antenna is 50’ from the house.

I recently picked up a "Balun Designs" choke that handles 300W, but I have not had a chance to test it. I have learned that if forget your choke once when using an OCF and you're miles from home, you do not do that again. This Balun Designs is half the size of the MyAntennas CMC-330-1K model. It is also about 1/3 the weight, for obvious reasons.

I have the MyAntennas 3-30 1k and I tested it at home inside the shack. I did not observe much difference with or without it as well. Again, probably due to that long run. My testing was cursory, so I need to try it more. Anyways, I purchased the MyAntenna in case the Balun Designs couldn't handle it, as well as probably using both (feed point / radio) when camping. ( I also leave six ferrite beads in the vehicle now too )

 

[W5lz]

Going only by the title of this thread, there should be absolutely no difference between a QRP antenna and a QRO antenna except for the size of various components to handle power levels. That 'QRP Antenna' thingy is pure nonsense. Nice advertising gimmick though...

 

[AK9R]

Except that "QRP antennas" are usually made of lightweight components, which are acceptable in QRP operations due to the low power, so that the antennas are lighter to back-pack into the remote spots that some QRPers like to operate from.

 

[prcguy]

This particular antenna system has been optimized for extreme portability, rapid deployment and very good on air performance. It can fit in your pocket and deploy in a minute or less and handle up to about 100w SSB. You cant't do that with any QRO rated antenna I know of. The components are just too big. I have a garage full of so called portable rapid deploy and military field HF antennas and nothing else comes close to this one for getting on the air fast and putting out a big signal with flea power.

I call it an antenna "system" because I usually build and supply them with tiny RG-316 Teflon or LMR100 coax with a small but effective ferrite common mode choke permanently in the feedline.

If you have a little more time, space and need VHF/UHF, here is another antenna I designed for rapid deployment that works great for QRP and QRO. The idea came about during Hurricane Katrina where first responders needed something that could do HF NVIS and 2m/70cm in a small portable package. See post #3 here: Portable Antenna Recommendations?

Going only by the title of this thread, there should be absolutely no difference between a QRP antenna and a QRO antenna except for the size of various components to handle power levels. That 'QRP Antenna' thingy is pure nonsense. Nice advertising gimmick though...