Making a Hexbeam Air Dielectric Coaxial Centerpost Feedline from Scratch

Most everybody I know has an NA4RR or K4KIO centerpost - myself included.   I have used a K4KIO centerpost in the past and had been happy with it.  I then decided to build center-post out of PVC pipe with the coax connections inside to prevent water from making contact with the foam core insulation and ruining the dielectric insulation between center conductor and shield.   I used it for about 6 months and decided to make a change to NA4RR's version and this has been on my home station for the better part of 2 years.  I did some research and captured the following equations to construct the equivalent of a 50 ohm (or as close to it) coaxial feedline.

For round coax, make the inside diameter of the outside conductor 2.302 times larger than the diameter of the inside conductor.

If the shield is square, and the inner conductor is still round, make the inside length of one side of the shield 2.134 times larger than the diameter of the inside conductor.

The derivation is as follows:

A coaxial transmission line has some inductance per unit length L$L$, and some capacitance per unit length C$C$. The conductors also have some (very low) resistance R$R$, and the dielectric has some (very low) conductance G$G$. Though it's important to note that these properties are distributed throughout the entire length of the transmission line, lumping them all together we can model a transmission line as:

The characteristic impedance is then:

The characteristic impedance is then:

Z0=R+jωLG+jωC−−−−−−−−√$$Z_0=\sqrt{\frac{R+j\omega L}{G+j\omega C}}$$

where:

• R$R$ is the resistance per unit length, considering the two conductors to be in series,
• L$L$ is the inductance per unit length,
• G$G$ is the conductance of the dielectric per unit length,
• C$C$ is the capacitance per unit length,
• j$j$ is the imaginary unit, and
• ω$\omega$ is the angular frequency.

If we assume a good conductor and dielectric, then R$R$ and G$G$ are negligible¹. The equation then simplifies to:

Z0≈LC−−√$$Z_0\approx \sqrt{\frac{L}{C}}$$

For round coaxial transmission lines, we can calculate C$C$ and L$L$ as:

C=2πϵ0ϵrln(D/d)

L=μ0μr2πln(D/d)$$L=\frac{{\mu }_0{\mu }_r}{2\pi }\ln (D/d)$$

where:

• d$d$ is the diameter of the inner conductor,
• D$D$ is the inside diameter of the outer conductor,
• ϵ0${ϵ}_0$ is the permittivity of free space, approximately 8.854⋅10−12F⋅m−1$8.854\cdot {10}^{-12}F\cdot m^{-1}$
• μ0${\mu }_0$ is the permeability of free space, approximately 1.256⋅10−6H⋅m−1$1.256\cdot {10}^{-6}H\cdot m^{-1}$
• ϵr${ϵ}_r$ is the relative permittivity of the dielectric, and
• μr${\mu }_r$ is the relative permeability of the dielectric.

Putting these together and simplifying we get:

For an air dielectric, ϵr${ϵ}_r$ and μr${\mu }_r$ are so close to 1 than you can further simplify:

Z0≈59.96ln(D/d)$$Z_0\approx 59.96\ln (D/d)$$

You can also see that only the ratio of the diameters of the conductors are relevant, so it doesn't matter what units of length are used, as long as they are the same.

We can solve for D/d$D/d$, and set Z0=50Ω$Z_0=50\mathrm{\Omega }$:

56.40ln(D/d)ln(D/d)D/dD/dD/d≈Z0≈Z059.96≈eZ0/59.96≈e50/59.96≈2.302

So in my case - 0.625" diameter and 1.5" diameter aluminum tubing are available from DX Engineering.  If I use average wall diameter for the outside tube and the OD of the inside tube, the ratio of D/d in this case is 2.3  which is really close. 

My plan is to put fiberglass sleeves around the bolts to insulate it from the outer shield along with nylon washers on the inside and outside.  Here is a scaled picture below

May Low Band Update

Top Band has effectively petered out to nil for ATNOs on the band.  I did manage to work TG9ADQ Guatemala on 160m-JT65 and immediately was confirmed in LOTW.  But 80m is still going - not gang busters as in February but a steady drip.  As far as highlights go on 80m:

The following DXpeditions were worked
E31A - Eritrea worked on 80m CW
HB9MFM - Switzerland worked with JT65 waiting LOTW
D44TWO- Cape Verde Islands - JT65  confirmed  LOTW
J5B- Guinea Bissau - OQRS - confirmed in LOTW
S79S  Seychelles - OQRS
TZ4AM Mali - confirmed in LOTW
GI4DOH Northern Ireland - confirmed in LOTW
5V7P-Togo - confirmed in LOTW
A25UK- Botswana - OQRS - confirmed in LOTW
FS/K9EL -Saint Martin - confirmed in LOTW

Confirmed count on 80m now up to 145.  I just received my 125 sticker in the mail from the ARRL.

Opportunities in the low bands this late spring and summer include:

May05 to May11 Andaman & Nicobar Is is VU4YC
May05 to May11 Jersey as MJ/PA1AW
May05 to May15 Fernando de Noronha is PY0NY (includes 160m)
May15 to May29 Galapagos is HC8/LW9EOC ()includes 160m)
May15 to Jul15 Minami Torishima JD1 - 80-10m
Jun24 to Jul01 Christmas I is VK9AA (ATNO on all bands)
Jul04 to Jul18 St Pierre & Miquelon FP/KV1J (80 and 160m)

April Top Band Update - Inching Closer

On April 12, I managed to work FJ/N2IEN (Saint Barts). UK9AA Uzbekistan  has confirmed by a card.  On 3/31 I managed to work Malta 9H1XT and this has just been confirmed in LOTW.  New received confirmations are RW2F Kaliningrad, FM/UT5UGR-Martinique, OG1X Finland, FJ/N2IEN

The inflection of the accumulation timeline has definitely flattened.  Confirmations now stand at 87.

The Best Months for the Lowbands

I have been collecting QSO data on 80 and 160m bands over the past few years and here is what I arrived at.  These are mostly unique DXCC's captured over a period of 4 years and based on my operating habits.   138/150 QSO's are unique for the 80m QSOs. 97/112 of the 160m QSOs were unique DXCC.  The plots below show that 160m is open less months than 80m.   With daylight hours being longer, the area coverage of darkness is much shorter for 160m long range QSOs during the warmer months.  This is data collected from the southeast US and there are likely regional variations with the west coast seeing less DX in the warmer month.  January and February in both cases are the best months, being the coldest  months of the year in SC.  For whatever reason there had been downturns in December, and I don't know why.

March Top Band Update

March 31 Update:

QSL's have slowed down to a trickle.  Ivory Coast TU7C ad El Salvador YS1YS confirmed in LOTW.   I was able to work HP1RN in Panama for #96 worked and Malta 9H1XT for #97 worked.  Confirmations now stand at 80 and still waiting for the 5U5R upload into LOTW..  There is still plenty of year left for DX Marathon.  DX Marathon now stands at 87 DXCC and 26 Zones.  I have discovered that the problem with my 160m antenna detuning SWR is because of water in the transformer.   I now have place drain holes in both the 160m and 80m transformers.

March 20 Update:

I decided to move the sweet spot of the 160m antenna to be more amplifier friendly as the AL1300 is finicky about SWR.  Afterwards, I tested the amps and the antenna tuners in a pileup envirornment chasing 5U5R.  With the tuner in bypass, there were no trips or faults and it appears my antenna tuner is fully mended.  Also Venezuela and  Aland Islands confirmed in LOTW.

March 19 Update:

At around 01:55 UTC I was finally able to work TU7C with my call now in the TU7C log.  I did manage to fry the antenna chain somewhere on the KAT500 as I was continually running 1200 Watts on the ALS1300.   I have to get this now repaired somehow.  I ordered a replacement for the AT2KD which I sold to a friend which will be a dedicated tuner for the ALS1300.


March 18 Update:

At around 02:40 UTC I was able to work TU7C and OH0Z in quick succession - still with the old amp.
Or at least I thought I did.  When the log was uploaded to Clublog - my call was not in the TU7C log

To my surprise however - I was able to chase down 5U5R on both 80m and 160m and it was confirmed in the on-line log.

At 10:58 worked YV1DIG and HK1MW.  YV1DIG uploads to LOTW.

March 17 Update:

I did want to see that my antenna is working properly.  I heard KL7/VE7ACN  calling CQ in the morning and made contact.   He appeared to be the only one strong enough in the RBN that I can work today.   He wan not picked up by W4KAZ skimmer in NC and he was just above my noise floor..  JA7QVI was also  just above the noise on my receiver.  The west coast skimmers were picking him up but none of the east coast skimmers.  I could not break out above his noise floor with 800W.  He was able to work stations on the east coast running more power.   My new amplifier the ALS1300 is enroute and promises to be QRV 3/18 - still  in time to try to help me work TU and 5U and maybe even S2 and 9N.

March 16 Update:

QSLs from HA8RM and HL5IVL received today.  The QSL from VO2NS was also received (needed for Zone 2 DX Marathon).  The confirmed count is now 76.  El Salvador YS1YS confirmed in LOTW. I tried to work 5U5R until his sunrise but no luck.   I did the same thin with TU7C - tried to work him till sunrise - also no luck.  

The following entities are expected to be active in top band during the month of March

5V7V Togo - mainly CW - March 4 to March 11 (I saw him spotted but tooweak to work)
JD1BLY  Ogasawara - March 7 to March 10 (did not see him spotted on the RBN)
YS1YS El Salvador - March 10-13 (worked)
9G5X Ghana - March 10-21
TU7C - Ivory Coast - March 9-19 (worked)
5U5R - Niger - March 9-21 (worked)
E51KTA - South Cook - March 10-19
S21KW - Bangladesh ATNO March 21-27
S21GM - Bangladesjh ATNO March 15-21
ZA - Albania - March 22-29
V633KS - Micronesia - March 19-April 3

The following entities are expected to transmit in top band during the month of April
3D2AG/P Rotuma - March 25-April 22
S79Z Seychelles - April 6-18
A25UK Botswana - April 25- May 6

The following entities are expected to transmit in top band during the months of May through July
FS/K9EL Jersey - May 1-15
MJ/PA1AW Jersey May 8-11
FP/KV1J - Saint Pierre and Miquelon - July 4-18

Top Band Update

Feb 25, 2017
Romania YO3APJ and Cyprus P3X confirmed in LOTW.  PJ4X and D4C also confirmed in LOTW. Now 73 confirmed in LOTW.  The QSL Pipeline is as follows:

YV5JDP
LZ1KFM
FY5KE
HA8RM received 3/15
OZ100DVI
UK9AA
HL5IVL received 3/15
TA3D

JY9FC

OG1X

FM/UT5UGR

S51V 

RW2F

V43Z

CO8LY

VO2NS (for Zone 2) received 3/15

Feb 14-15, 2017
Worked 2 new ones OG1X Finland and FM/UT5UGR Martinique.   OA4TT confirmed in LOTW and the card from CT9/DK7YY came

Feb 13, 2017
Worked OA4TT early in the morning - band noise really low but no one on.   worked HR5 last night and confirmed in LOTW.

Feb 12, 2017
PACC contest was on and I needed PA for 160m DXCC - so I worked a whole bunch just to make sure I had one.   FM5BH was there again, get a pileup and runs off.   I worked AK early in the morning on JT65 for #77.  KL7J just confirmed me in LOTW.

Feb 11, 2017
G3PQA England confirmed for #58 via card, IQ9UI Italy confirmed for #59 via card.

Feb 10, 2017
SV3RF Greece confirmed in LOTW #57.  Band has been pretty dead of late and only regional contacts on JT65.  The only contacts made last night were UR0MC into the Ukraine, VO1HP into Newfoundland, and Alex HC2AO into Ecuador.

Feb 09, 2017
Belarus EU4A and Dodecanese SV5DKL confirmed in LOTW for #55 and #56.  JY9FC worked after a small window of propagation opened.

Feb 08, 2017
Was on JT65 for the first time in 2017.  New ones were on the waterfall like CO, OX, but I was asleep.  I did work KU4XO Matt who saw me calling CQ.  S9+ static crashes on the TX antenna.  Still high on the RX antenna.  No new ones today

Feb 07, 2017
No new ones today - TL8TT had a pretty good signal but could not bust the EU wall.  I did work Alex HC2AO who I have not worked since 2015

#54 VK3IO confirmed in LOTW

Feb 06, 2017
Only new one today is Malta 9H3WHX - worked him twice - but weak copy - WFWL.  K7BG Montana has confirmed in LOTW on 160m - giving me 48/50 states.  Only missing NE and AK.  I tried to work ZC4A on 80m for an ATNO but no luck - only 1 day to go on that DXpedition.  Band died after the Super Bowl finished and has not woken up yet.  No signals from Asia.  NA skimmers only hearing AA1K, K2AV and other US stations.

Feb 05, 2017
Only new one so far just before midnight is P3X Cyprus.   Tried to work an FM5 - no luck there.   Then EY8MM Nodir from Tajikistan started calling CQ on 19.5  and built up a pileup real quick - no luck there either.  Will take a break and do some work for work (real work - this is just a hobby).  After the break, I came back into the shack to work TA3D and 4X4NJ.  They were loud but can't hear. A bit later propagation improved and I was able to work both TA3D and SV5DKL.  In the morning VY2ZM, HI8A W0VTT (Zone 3), K3RS (Zone5) and ZF9CW (needed for 160m DX Marathon).

#53 LX7I has confirmed in LOTW

Feb 04, 2017
Worked 5 new ones today - OZ100DVI - Denmark, EU3A Belarus,  UK9AA Uzbekistan and in the morning grey line HL5IVL South Korea.  Then later around 4:45 UTC SV3RF Greece.  OA4TT was loud and workable, had a pileup but could not get through.   OA4TT faded as plenty of sun in Peru by 1100 - Jack replied to me on the top band reflector that he was only working JA's and ignoring W's.  Maybe tomorrow

Joe-san JA1LZR was loud and workable - and I was able to work him.  He and I exchanged pleasantries on the top band reflector.  Nice opening to Japan as I was also able to work JH1HDT.  I can hear JA8ISU real well 569 579 but he could not hear me - one way propagation to his QTH. JA7QVI was a different story - was able to work him as he was the loudest I heard this morning.

#51 DJ8QP confirmed LOTW
#52 JA7QVI confirmed in LOTW

Feb 03, 2017
Large pileup on TL8TT - an Iralian DXpedition to Central African Republic.  The Op working was very good in copying callers.  Unfortunately I could not get through.  They will be there until the 14th of  February.  I worked ZF9CW with 75 watts - he was really strong.  Alaska was seen on JT65.  JAs heard in the morning was only JA1LZR amd was perhaps 339.

#48 S01WS confirmed LOTW
#49 7Z1SJ confirmed LOTW
#50 EI4KF card arrived

HA8RM card sent

Feb 02, 2017
Finally a QSO with Hungary - Peter HA8RM.   Good conditions into Eastern Europe at 4:30 UTC.  I heard Peru OA4TT in the morning copiable for a bit then faded

Jan 30, 2017
Conditions tonight are pretty good with low local QRN and just worked F6ARC for grins.  Also tested the RBN to see how my signal is reported by various skimmers.

Received a couple of topband QSL cards in the mail which was a good surprise:

V26M - Antigua and GW3YDX - Wales,  This brings total confirmed in cards and LOTW to 47.  Also the western states of NV and WA confirmed in LOTW bringing my state total to 47.  MT was also worked but tat confirmation is pending.   That would bring my state total to 48

The following have either card or LOTW confirmations pending

New 80m Contacts

Inverted L over FCP working well - heard VP6EU twice early 5:00 AM and 5:30 AM local time - no pileup.  Worked 5J0NA last night and KL7RA a few nights ago in the contest.  PJ7/OH2IS is also a new one on 80m.

ARRL International DX Contest CW 2017

Day 2 Highlights

Fantastic conditions with low band noise allowed me to listen on the TX antenna.  This also allowed me to work 6 new stations.   Worked entities now stand at 92.  New ones include V43Z - Saint Kitts and Nevis, VP5K Turks and Caicos, CO8ZZ Cuba, RW2F Kaliningrad, D4C Cape Verde Islands, A45XR, Oman, and fill ins for previous stations that will not QSL Bonaire PJ4X and Slovenia S51V.  Had I been up and out of bed an hour earlier I would have caught Chile and Uruguay at their sunrise.  Oman A45XR, VP5K Turks and Caicos and Kuwait 9K2NO confirmed in LOTW.

Day 1 Highlights

I had planned only to work 160m but installed the 80m antenna with the FCP regardless.   It turned out that was a good thing.  I had never worked AK very well on 80m CW.  KL7RA (Rich now SK but call sign still lives as a club station) was active in the low bands and I was able to work him on both 80m and 160m.  Regarding the antenna installation, I had purchased 80 feet of 12 gage stranded THHN thinking I would need to prune a few feet.  It turned perfect without trimming.  How lucky can one get?  Resonance point was at 3530 and below 2 in SWR from 3500 to 3600 - so it had a 100 kHz bandwidth below 2 in SWR.  I also worked ZL3X - a contest station in New Zealand on 80m - a new band slot.

I did not work any ATNOs on 160m but I was able to work stations previously worked  that will not QSL - namely Slovenia and PJ4.  So they effectively became ATNO's.  I am still looking for a Cuban statioon on 160m - any mode.  I also worked Romania YO3APJ - which previously could easily have been a busted QSO.   I will work that entity again tonight to make sure I have a solid QSO.

Worked DXCC currently stands at 85.  The pipeline of cards is still full and I have 8 cards in hand with 60 more confirmed in LOTW.

ND7J's Run to Challenge 1000

ND7J Bob was first licensed in March 2013 and his goal was to reach 5 Band DXCC and Challenge 1000 in 4 years.  As can be seen from the plot below, he achieved one of those stated goals - a clear run from zero to 1000 in less than four years.  Bob has been supplying me QSL information over the years and the green triangles are  his QSL confirmed data.   The blue and red circles and squares are my QSL confirmed data.

When do the 160m Fish Bite?

The following chart is data from KU4XO from over 100 Contacts

This is the data I have collected so far from about 95 worked entities and the list continues to grow.  I will update the chart below as time goes by.  The similarities are remarkable.

ATNO #304 ZC4A UK Bases in Cyprus

DX Watch indicated that they started to come in on 17m at 18075.  Quickly ran down to the shack and worked the station.   They were barely above the noise at 1300 UTC.  They first copied me as NY4I but then came back with the correct call after a couple of exchanges.

ZC4A has just now confirmed in LOTW.

DXpeditions 2017 Which Plan to Transmit on Top Band - February

A5A Bhutan February 10-15, 2017 by Zoro JH1AJT
5J0NA San Andres and Providencia Feb 13-27
TX5T Austral Island Feb 13-March 3
V3 Belize Feb 15-21
SX5R Dodecanese Feb 24-26 ARRL 160m SSB
 Others to follow in March - will update then