Portable Flag Antenna for MF/HF Radio Direction Finding

Portable Flag (Terminated Loop) Antenna For MF/HF Radio Direction Finding

By Don Kirk (wd8dsb)

This website contains basic details on the portable flag antenna I designed for tracking down RFI (radio frequency interference) on MF (medium frequencies) and HF (high frequencies), as well as some tips and tricks on its use and frequently asked questions. Full details about my antenna are in the March 2021 issue of QST and here is a link to the full article with the permission of the ARRL Portable Flag article QST March 2021. Also note that DX Engineering is now selling an antenna kit based on my design that incorporates fiber glass spreader arms and here is a link to the DX Engineering website (note: to keep the cost low, I do not make any money on the sale of this antenna by DX Engineering). https://www.dxengineering.com/parts/dxe-noiseloop

Note: The DX Engineering version of my portable flag uses 48 inch spreader arms versus the 56 inch long spreader arms that I used. DX engineering said that to keep cost as low as possible "We have 48” fiberglass as stock, and cutting our 90” pieces to 56” would give us a 34” piece of scrap. It also helps greatly on shipping." The change in spreader arm length causes a slight reduction in gain of approximately 2 to 3 dB with negligible impact on the antenna pattern. John W1FV reports the DX Engineering version of my portable flag is 42 inches x 21 inches versus my 48 inches x 24 inches, and it appears to be working well.

Important note about nearby objects altering the pattern of the antenna:
Skewing of the pattern can happen if you are located close to other metallic objects that can reflect or re-radiate the signal or cause part of the antenna to couple to the metallic object. Always make sure you use the portable flag away from other objects to avoid pattern skewing and this is typical advice when using any kind of direction finding antenna. Use the portable flag when out in the clear for best results. It's easy for the null to get skewed and when it does not agree with the peak direction, the peak becomes much more reliable for direction finding.

I have sometimes noticed the portable flag develops two nulls when standing very close to my house as an example because a minor lobe develops where the null should be which makes it appear there are two nulls (possibly due to interaction with my other antennas or house wiring, etc.). This only happens when standing very close to my house (like 10 feet away), and when this happens it's just easier using the peak of the antenna to determine general direction but I would suggest you move away from objects so this does not happen (you can also use the minor lobe that has replaced the null or split the difference between the 2 nulls but I prefer to just trust the peak when this condition exists). I have never had this problem when standing out on the sidewalk in front of homes as an example, and that's where I typically obtain my readings when out direction hunting.

DX Engineering also designed a very nice preamp for use with my portable flag which I field tested during the development stage of the preamp. The preamp provides 30 dB of gain and also has attenuators of 10 and 20 dB which are selectable for a total of 30 dB of attenuation. It also has a selectable AM broadcast band rejection filter. Here is a link to the DX Engineering website (note: I do not make any money on the sale of DX Engineering products). https://www.dxengineering.com/parts/dxe-nl-pre-att-1

Basic features of the portable flag

• Cardioid pattern which is unidirectional.

• Designed to easily fit in the backseat of my car.

• Typical front to back ratio 12 to 21 dB (2 to 3.5 S units) which is more than adequate for direction finding.

• Designed for 50 ohm systems (SWR measured 1.5 to 1 or less all the way up to 30 MHz when using wire wrap wire for the transformer).

• Normally requires preamp (see preamp comment in sketch below as well as in text that follows below)

Youtube Demonstration Video : https://youtu.be/yIWY2KwoZ8w

Design that I have been using in the field shown below (very basic build).

Antenna wire dimensions

24 inches tall

48 inches long

Termination Resistor: 680 ohms

Matching Transformer: BN73-202, primary 3 turns, secondary 12 turns
( Transformer wound with 30 gauge wire wrap wire loss test results = 0.62 dB @ 138.5 KHz, 0.18 dB @ 500 KHz, 0.59 dB @ 30 MHz)

Feedline: RG-174A/U (10 feet) plus 3 feet for jumper between preamp and receiver.

Appropriate preamps

W7IUV (broadband preamp with gain of 20 dB)

W1FB variable preamp (160 and 80 meter preamp with max gain in excess of 50+ dB)

KD9SV VLN variable preamp (160 and 80 meter preamp with max gain of 40 dB)

Advanced Receiver Research has a nice line of MF/HF preamps that look interesting such as their model P0.1-30/20VD or their model P0.5-30/20VD. Both models provide 20 dB of gain but the first model goes down to 100 KHz and costs slightly more than the second model which goes down to 500 KHz.

DX Engineering offers a 30 dB preamp designed specifically for use with the portable flag and it includes selectable attenuation and a selectable AM broadcast band rejection filter. Its part number is DXE-NL-PRE-ATT-1.

Note: I just use a 9 volt battery to power the W1FB and W7IUV preamp when out in the field.

Portable Terminated Flag shown below with and without 46 inch long extension

Note: while out DFing on foot I have found the 46 inch extension is not really needed, and no longer typically use the extension (use it as shown below). I sometimes still use the 46 inch extension if doing stationary experiments, etc.

Below is a graph showing front to back ratio vs. elevation angle on 160 meters, 30 meters, and 10 meters.

Below are the 160 meter plots showing the vertical plot and then the horizontal plots at 1, 20, 33, and 45 degree elevations.
(680 ohm termination resistor)

Below are the 10 meter plots showing the vertical plot and then the horizontal plots at 1, 20, 33, and 45 degree elevations.
(680 ohm termination resistor)

Below is a plot showing the Front to Back Ratio if using a 820 ohm termination resistor. Note the modeled front to back ratio is not as good at low elevation angles compared with using the 680 ohm resistor I use with my portable flag, but in practice an 820 ohm termination works well as does the 680 ohm termination resistor I use. More recent testing indicates a 820 ohm resistor offers a slight increase in front to back ratio on 10 meters whereas the 680 ohm resistor offers a slight front to back ratio advantage down in the AM broadcast band, and either value of termination resistor offers overall good performance since the difference noted between the two termination resistors is barely noticeable.

Plot of Gain Versus Frequency (modeled using 4NEC2)

Tips and Frequently Asked Questions

Question: Where should attenuators be placed when needed?

Answer: When using a preamp with the portable flag, the use of any attenuators should come after the preamp so the signal to noise ratio is not degraded. The portable flag has negative gain so the signal reaching the preamp is small and placing attenuation between the portable flag and the input of the preamp causes the noise figure of the preamp to become a dominate factor which degrades the signal to noise ratio. To avoid signal to noise degradation use attenuators on the output side of the preamp.

Question: I sometimes notice a slight skewing of the pattern in which the null and the peak of the antenna do not yield the exact same signal direction.

Answer: skewing of the pattern can happen if you are located close to other metallic objects that can reflect or re-radiate the signal or cause part of the antenna to couple to the metallic object. Always make sure you use the portable flag away from other objects to avoid pattern skewing and this is typical advice when using any kind of direction finding antenna. Use the portable flag when out in the clear for best results. It's very easy for the null to get skewed and when it does not agree with the peak I then rely much more on the peak.

Question: I used enamel coated wire instead of wire wrap wire when winding the transformer and notice that my maximum SWR is closer to 2.0 to 1 on the higher frequencies versus 1.5 to 1 as mentioned in the article.

Answer: This is typical when using enamel coated wire for the transformer in place of the wire wrap wire in this application. Since the run of coax is so short the slightly higher SWR is of no significance as it causes very little extra feed line loss and it does not degrade the performance of the portable flag.

Question: How do I connect the transformer?

Answer: The transformer secondary has 12 turns and it connects to the antenna, whereas the transformer primary has 3 turns and it connects to the coax.

Question: "For the binocular cores, do you count one pass through one hole as one turn?"

Answer: No, one complete turn on the binocular core requires the wire to pass through both holes in the core one time as shown in the picture below.

Question: "You mention soldering the lug to the body but I’m unclear what you mean by that. The coax shield still solders to the outside of the plug and the center conductor to the plugs center pin?"

Answer: Yes, the coax shield is still soldered to the outside of the plug (its ground lug), and the center conductor is soldered to the center pin. Over the years I have found that the RCA plug develops an unreliable ground connection, and this is because the ground lug is staked or press fit into its body and becomes lose over time. I therefore always solder the ground lug to the main body as shown in the picture below and this makes the RCA plug a very reliable connector. I occasionally check this solder joint to make sure a crack has not developed over time, and if it does I just heat the joint back up to reflow the solder.

Question: You used 14 gauge bare copper wire, I have some 16 gauge PVC covered solid wire handy and I can easily get 14 stranded. Will these different types of wire be OK to use?

Answer: You can use whatever wire you have available. Plastic coated wire will work just fine, a different gauge wire will just fine, and stranded wire will work just fine in place of the 14 gauge bare copper wire that I used.

DXE Engineering Preamp model DXE-NL-PRE-ATT-1 data shown below

AM Broadcast Band Filter Response Curve

Preamp Gain Vs. Frequency Plot

Below are some examples showing how others have built my portable flag using various construction techniques.

P.S. thanks to Earl (K6SE) who is now a SK for originally introducing large stationary Pennants and Flags to the ham radio community.

Antenna designed by Don Kirk (wd8dsb). Website designed and maintained by wd8dsb 18FE20

Design may be copied for personal use, but not for commercial sales except for sales by DX Engineering.